A series of four explosions struck London's public transportation system Thursday in what Prime Minister Tony Blair (search) called a coordinated series of "barbaric" terrorist attacks, most likely timed to coincide with the opening of the G-8 summit in Scotland.

At least 37 people were reported dead and at least 700 were wounded, according to officials.

"This is a very sad day for the British people," Blair said in an afternoon address from 10 Downing Street. "We will not be terrorized."

Deputy Assistant Commissioner Brian Paddick of the Metropolitan Police (search), the capital's police force, told a press conference that there had been seven confirmed fatalities in the first subway blast, 21 in the second and five in the third, which may have affected three trains near the Edgware Road train station.

In the fourth explosion, which involved a double-decker bus, he said there were also fatalities, but no firm numbers.

"This clearly was a callous attack on purely innocent members of the public, deliberately designed to kill and injure innocent members of the public," Paddick said.

He added that British officials had received no prior warning, nor did they have any advance intelligence about the attacks.

As the city's public transit system ground to a near-halt, buses were used as ambulances and an emergency medical station was set up at a hotel.

Rescue workers, police and ordinary citizens streamed into the streets to help. At the scene of several blasts, specialist emergency workers in orange hazardous-materials suits searched for evidence of biological, chemical or nuclear agents.

At the request of Queen Elizabeth II, the Union Jack flying over Buckingham Palace was lowered to half-staff.

Blair gave a brief televised address from the G-8 summit taking place in Scotland before leaving for London. He concluded the attacks were terrorist actions.

"It's important ... that those engaged in terrorism realize that our determination to defend our values and our way of life is greater than their determination to cause death and destruction to innocent people and a desire to impose extremism on the world," a clearly shaken Blair told the world.

"Whatever they do, it is our determination that they will never succeed destroying what we hold dear in this country and in other civilizations in the world."

Later, at Downing Street, Blair said acts by Islamic terrorists should not reflect negatively on Britain's large Muslim population.

"We know that these people act in the name of Islam, but we also know that the vast and overwhelming majority of Muslims, here and abroad, are decent and law-abiding people who abhor this act of terrorism every bit as we do," he said in London after meeting with emergency-response officials.

"When they try to intimidate us, we will not be intimidated … we will not be changed," Blair added. "When they try to divide our people our weaken our resolve, we will not be divided and our resolve will hold firm. We will show by our spirit and dignity ... our values will long outlast theirs."

G-8 leaders condemned the attacks.

The U.N. Security Council also condemned the attacks in a unanimous resolution, drawn up by Britain, which expressed sympathy for the victims. It urged all nations to help bring those responsible to justice and expressed the council's determination to combat terrorism.

"This meeting of the council is very important to the United Kingdom," U.N. Ambassador Emyr Jones Parry said before walking into the council chambers.

Much of Europe went on alert. Italy's airports raised their alert level to a maximum. The Czech Republic, Hungary, Russia, the Netherlands, France and Spain increased security at shopping centers and airports and on railways and subways.

The U.S. State Department said there were no reports yet of Americans hurt in the blasts. The department set up a task force and call center for those with relatives and friends in London. The toll-free number is 1-888-407-4747.

For more, go to these FOX News partners: Sky News | The Times of London | The Sun

Unknown Group Claims Responsibility

A previously unknown group, "Secret Group of Al Qaeda's Jihad in Europe," claimed responsibility in the name of Al Qaeda for the blasts, saying they were in retaliation for Britain's involvement in Iraq and Afghanistan.

The group claimed the attack in a Web-site posting and also warned Italy and Denmark to withdraw troops from Iraq and Afghanistan.

Arabic satellite television station Al-Jazeera said it had received a phone call from the same group claiming responsibility.

Neither claim could be verified.

The Web site carrying the posting is popular with Islamic militants, according to Elaph, a secular Arabic-language news Web site, and Der Spiegel magazine in Berlin, which published the text on its German- and English-language Web sites.

"Rejoice, Islamic nation. Rejoice, Arab world. The time has come for vengeance against the Zionist crusader government of Britain in response to the massacres Britain committed in Iraq and Afghanistan," said the statement, as translated by The Associated Press in Cairo. "The heroic mujahedeen carried out a blessed attack in London, and now Britain is burning with fear and terror, from north to south, east to west."

FBI and Justice Department officials said it was still too early to conclude that the London attacks were the work of Al Qaeda, but other experts were more certain.

"It has all the earmarks of Al Qaeda," Dennis Ross, special Middle East envoy for Presidents George H.W. Bush and Bill Clinton, told FOX News. "This has probably been planned for a long time. They probably had some sleeper cells in the U.K. for some time. It really looks very much like an Al Qaeda operation, and I think we have to assume they're responsible."

One U.S. counterterrorism official emphasized to FOX News that in previous Al Qaeda attacks, such as on Sept. 11, 2001, and the embassy bombings in Africa in 1998, there was no immediate claim of responsibility.

The group making the London terror attack claims has not been heard of before.

There are obvious comparisons to the bombings of four commuter trains in Madrid on March 11, 2004, which killed 191 people. But the American official noted that the Madrid attacks were carried out by a group that was only inspired by Al Qaeda, not one under the command of Usama bin Laden himself.

What was notable about Thursday's attacks was their extreme coordination, their timing during rush hour for maximum casualties and their targeting of the transit system, counterterrorism officials told FOX News.

"It was chaos," said Gary Lewis, 32, who was evacuated from a subway train at King's Cross station. "The one haunting image was someone whose face was totally black and pouring with blood."

The attacks came a day after London was awarded the 2012 Olympics (search) and as the G-8 summit (search) was getting underway in Scotland.

A spokesman for the Olympic committee said it still has full "full confidence" in London as the host of the 2012 Games.

From Singapore, where he had been for the Olympic selection, Greater London Authority Mayor Ken Livingstone (search) said: "I want to say one thing, specifically to the world today — this was not a terrorist attack against the mighty and the powerful."

"It was not aimed at presidents or prime ministers," the former Trotskyist said. "It was aimed at ordinary, working-class Londoners, black and white, Muslim and Christian ... young and old … that isn't an ideology, it isn't even a perverted fate, it is an indiscriminate attempt at mass murder."

"They seek to divide London, they seek Londoners to turn against each other ... this city of London is the greatest in the world because everybody lives side by side in harmony. Londoners will not be divided by this cowardly attack."

Livingstone also had words specifically for the terrorists: "I know that you personally do not fear to give your own life in exchange to taking others ... but I know you do fear you may fail in your long-term objective to destroy our free society ... in the days that follow, look at our airports, look at our seaports and look at our railway stations ... you will see that people from the rest of Britain, people from around the world, will arrive in London to become Londoners, to fulfill their dream and achieve their potential … whatever you do, however many you kill, you will fail."

G-8 Leaders Stand United Against Terrorism

Blair, who was hosting the world's most powerful industrial leaders at Gleneagles, Scotland, left the G-8 summit to meet with police and other officials but said the rest of the leaders would remain. The G-8 gathering is focusing on climate change and aid for Africa — but Iraq has largely been left off the agenda.

"Each of the countries around that [G-8] table have experience with the effects of terrorism and all of the leaders ... share our complete resolution to defeat this terrorism," Blair said in his address Thursday. "It's particularly barbaric this has happened on a day when people are meeting to try to help the problems of poverty in Africa, the long-term problems of climate change and the environment."

The G-8 leaders later issued a statement of their own. Blair, flanked by President George Bush on one side and French President Jacques Chirac on the other, read the statement on a stage full of the other G-8 leaders in a sign of solidarity.

"Those responsible have no respect for human life. We are united in our resolve to confront and defeat this terrorism that is not an attack on one nation but on all nations and on civilized people everywhere. We will not allow violence to change our societies or our values, nor will we allow it to stop the work of this summit," the statement read.

"The terrorists will not succeed. Today's bombings will not weaken in any way our resolve to uphold the most deeply held principles of our societies and to defeat those who would impose their fanaticism and extremism on all of us. We shall prevail and they shall not."

Bush later told reporters that the people of London have America's "heartfelt condolences" and said he appreciates Blair's steadfast determination and strength.

"He'll carry a message of solidarity with him" as he leaves the G-8 summit for London, Bush added. "I was most impressed by the resolve of all the [G-8] leaders in the room and that their resolve is as strong as my resolve. ... We will not yield to the terrorists. We will find them; we will bring them to justice."

Bush said there's a clear contrast between the work being done at the G-8 summit and the goals of the terrorists responsible, "those who've got such evil in their heart that they will take the lives of innocent folks."

"The War on Terror goes on," he added.

NATO Secretary General Jaap de Hoop Scheffer called the attacks "appalling."

"I know I speak on behalf of all NATO allies when I express our sympathy for and solidarity with the British people. There can be no justification for such heinous crimes," he said in a statement. "I condemn in the strongest terms these attacks, which underline the need for the international community and members of the alliance to remain united in the fight against terrorism."

Jamie Rubin, a former foreign affairs specialist under former Secretary of State Madeleine Albright, told FOX News from London that the attack may recreate some of the unity of world leaders that was seen right after the Sept. 11, 2001, attacks on the United States.

"For now, some of that sense of solidarity that is in the War on Terror ... is restored," Rubin said. "The political sense of solidarity has been lost a bit and I think, ironically, this tragedy may restore it."

Blasts Hit Buses, Subway System

Police said incidents were reported at the Aldgate station near the Liverpool Street railway terminal, Edgware Road and King's Cross in north London, Old Street in the financial district, Russell Square in central London, near the British Museum, Aldgate Station and Leicester Square, which is the equivalent of New York City's Times Square. A police official also told reporters there was an incident on a bus in Tavistock Place.

The first explosion on a tube train in a tunnel on the east side of London's financial district occurred at 8:49 a.m. local time. The second blast went off on a train sitting in the Edgware Road station at 9:33 a.m. The third occurred at 9:40 a.m. on a tube train between Russell Square and Kings Cross stations. The fourth blast happened at 9:50 a.m. on a double-decker bus in Tavistock Square near Russell Square in central London.

Sir Ian Blair (search), commissioner of the Metropolitan Police, said officials had found traces of explosives at one of the blast sites.

Bradley Anderson, a subway passenger, told Sky News that "there was some kind of explosion or something" as his train reached the Edgware Road station in northeast London.

"Everything went black and we collided into some kind of oncoming train," Anderson said.

Simon Corvett, 26, who was on an eastbound train from Edgware Road station, said: "All of the sudden there was this massive, huge bang."

"It was absolutely deafening and all the windows shattered," he said. "There were just loads of people screaming and the carriages filled with smoke.

"You could see the carriage opposite was completely gutted," he said. "There were some people in real trouble."

Jay Kumar, a business owner near the site of the blast that destroyed the bus, said he ran out of his shop when he heard a loud explosion. He said the top deck of the bus had collapsed, sending people tumbling to the floor.

Many appeared badly injured, and bloodied people ran from the scene.

"A big blast, a big bomb," he said. "People were running this way panicked. They knew it was a bomb. Debris flying all over, mostly glass."

"I was on the bus in front and heard an incredible bang, I turned round and half the double-decker bus was in the air," Belinda Seabrook told Press Association, the British news agency.

Home Secretary Charles Clarke (search), Britain's top law enforcement officer, said there were "terrible injuries."

Clarke, in a later address before English lawmakers, said above-ground train service is subject to "substantial delays," but airports are operating normally.

"People are strongly advised not to travel into central London as the emergency services must be allowed to work as effectively as they can," Clarke said, adding that there were three explosions in the subway and one on a bus.

Pope Benedict XVI deplored the "terrorist attacks," calling them "barbaric acts against humanity," and said he was praying for the families of the victims.

Americans Urged to be Vigilant

U.S. officials said they had no intelligence that suggested similar attacks were planned for the United States; there are no plans currently to raise the terror alert system. But the U.S. rail system was placed on orange alert. Click here for that complete story.

"The United States government is raising the threat level from code yellow to orange, targeted only to the mass transit portion of the transportation sector," Homeland Security Michael Chertoff said in an afternoon press conference.

While there was no credible information indicating an attack in the United States, Chertoff added that, "we know the tactics and methods of terrorists, as demonstrated by the terrorist attacks last year in Madrid."

To the people of London, Chertoff said: "America stands with you in this time of crisis in attempts to help in any ways possible."

Despite terrorists' attempts to shake the will of free people around the world, Chertoff added, "we have a more powerful arsenal. It includes our resolution and resilience ... to keep our citizens and our allies safe and track down those who perpetrate incidents like this and to bring them to justice."

Bomb-sniffing dogs and armed police officers were sent to patrol Washington's subways and buses Thursday. About 1.2 million people a day ride Washington's buses and trains. The U.S. Capitol also tightened up security.

FOX News' Catherine Herridge, Scott Norvell, Anna Persky, Liza Porteus, Teri Schultz and The Associated Press contributed to this report.

For more, go to these FOX News partners: Sky News | The Times of London | The Sun