Two weeks to the day after terrorist attacks in London killed a total of 56 people, explosions struck three more Underground trains and a bus at lunchtime Thursday.

Metropolitan Police commissioner Sir Ian Blair confirmed that three explosions occurred in the subway on trains near the Oval station on the Northern line, Warren Street station on the Northern and Victoria lines and Shepherd's Bush station on the Hammersmith and City line, and that a fourth explosion took place on a bus traveling in the Hackney neighborhood of east London.

None of the presumed bombs appeared to have detonated properly. Officials were hoping that the leftover explosives would provide a wealth of forensic evidence to help investigators hunt down whoever planted them.

"Obviously, this has been another difficult day," said Sir Ian Blair during a press conference Thursday afternoon. "Clearly, the intention must have been to kill. You don't do this with any other intention, and I think the important point is that the intention of the terrorists has not been fulfilled."

Police officials said it was too early to tell whether the failed bombings were the doings of an Al Qaeda or affiliated cell.

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Police said the four explosions, spread over a large area of central London, had injured one person.

One Sky News correspondent reported that officials said the explosive device on the bus appeared to be made of TATP, the same highly unstable material used in the July 7 bombings. A large area around the bus had been cordoned off.

Police said an armed police unit had entered University College Hospital, near the Warren Street station. Press Association, the British news agency, said the team arrived shortly after an injured person was carried in.

Sky News TV reported that police were searching for a black or Asian male, 6 feet 2 inches tall, wearing a blue shirt with wires protruding out a hole in the back. "Asian" in Britain refers to people with origins in the Indian subcontinent.

Scotland Yard said there had been two arrests, one at 10 Downing Street outside the prime minister's residence, but stressed that the arrests were not related.

Prime Minister Tony Blair said Thursday that officials hoped that London would return to normal "as soon as possible."

"We can't minimize incidents such as this," Tony Blair said during a news conference with visiting Australian Prime Minister John Howard, adding that such attacks were meant "to make people worried and frightened and taking responsibility off the shoulders of people who engage in these acts."

Blair said the police and security services were "fairly clear" on what happened and what the next steps were, and that it was "important to respond by keeping to our normal lives."

"People have seen our country react to terrorist attacks that are meant to make people worried and scared and react, and people have reacted with great dignity. ... It's not going to change us … Our reaction's got to be the same" as it was after the July 7 attacks, Tony Blair added.

He said it was too early to speculate on who might be responsible for the July 21 attacks.

Howard told Tony Blair that Australia would remain a "steadfast partner" with Britain in the fight against terrorism.

"Terrorism is an enemy to all free people," Howard said. "Terrorism is not just about individual circumstances and individual events."

A Criminal Act

Sir Ian Blair said "attempts" to set off explosives were made each location, but he would not comment directly on any part of the investigation.

"This is an incredibly fast-moving scene, a very fast-moving set of events," he said. "The main message is now that London has gone past this point again, we're back to business."

Although the Hammersmith and City, Victoria and Northern lines were still closed, shuttles were running to aid Underground commuters, Blair said.

Officials, including London Mayor Ken Livingstone, urged Londoners to report any suspicious activity and to send information to the Web site www.police.uk.

Sir Ian Blair also urged patience and restraint, obliquely urging no retaliation against Britain's large Muslim community. Islamic extremists are believed responsible for the July 7 attacks.

"No community should be smeared with responsibility for these matters. These are criminal acts, and we are in pursuit of a set of criminals in relation to it," Blair said.

'People Were Panicking'

After the explosions, the Warren Street, Shepherd's Bush and Oval stations were evacuated. Emergency services personnel were called to the stations. There were reports of a nail bomb and a backpack exploding at Warren Street, but those reports had not yet been confirmed.

Police in chemical protection suits were seen preparing to enter the Warren Street station, but no apparent chemical agents were involved.

There was also an incident at 10 Downing Street, where the British prime minister's offices are located. Two police officers trained their guns on a man before leading him off the premises; his shirt was unbuttoned in the front.

Some witnesses said they heard gunshots at Warren Street, but police believe those noises were actually detonators going off for bombs that failed to explode.

"People were panicking. But very fortunately the train was only 15 seconds from the station," witness Ivan McCracken told Sky News.

McCracken said he smelled smoke, and people were panicking and coming into his carriage. He said he spoke to an Italian man who was comforting a woman after the evacuation.

"He said that a man was carrying a rucksack [backpack] and the rucksack suddenly exploded. It was a minor explosion, but enough to blow open the rucksack," McCracken said. "The man then made an exclamation as if something had gone wrong. At that point everyone rushed from the carriage."

"I was in the carriage and we smelled smoke — it was like something was burning," said Losiane Mohellavi, 35, who was evacuated at Warren Street. "Everyone was panicked and people were screaming. We had to pull the alarm. I am still shaking."

Stagecoach, the company which operates the stricken No. 26 bus, said the driver heard a bang and went upstairs, where he found the windows blown out. The company said the bus was structurally intact and there were no reports of injuries.

Closed-circuit TV cameras on Hackney Road showed the No. 26 bus immobilized at a stop with its indicator lights flashing. The area around the bus had been cordoned off.

Keith Roberts, who keeps a shop close to where the bus was sitting empty, said the bus driver was speaking to police and that not only the bus, but nearby houses, had been evacuated.

Roberts and a witness from the Shepherd's Bush station who called in to Sky News both said they were told by police to get off their cell phones in order not to accidentally set off any unexploded bombs.

Thursday's incidents were hauntingly similar to the blasts two weeks ago, which involved three simultaneous explosions on three Underground trains, followed by a blast on a bus an hour later. Those bombings took place during the morning rush hour and were even closer to the center of London.

London Ambulance said it was called to the Oval station at 12:38 p.m. and Warren Street at 12:45 p.m. on July 21. The July 7 attacks began at 8:51 a.m.

Prime Minister Blair had already been scheduled to meet with various British intelligence agencies Thursday. He canceled a visit to a school in east London.

President Bush was briefed on the London incidents, according to White House spokesman Scott McClellan, who said U.S. officials were "monitoring the situation closely.'"

Washington, D.C., officials said the city's Metro, on a higher alert since the July 7 London bombings, would see even more of an increase in security and bomb-sniffing dogs.

New York Mayor Michael Bloomberg said police would begin random searches of packages and backpacks carried by people entering subway stations. Authorities said it was also possible that checks would begin on city bus and commuter-train passengers.

There was no plan to change the U.S. terror threat level.

The latest London explosions came as Pakistani intelligence officials said authorities were seeking the former aide of a radical cleric in Britain in connection with the July 7 bombings.

The officials said British investigators asked Pakistani authorities to search for Haroon Rashid Aswat (search), who reportedly had called the London suicide bombers on his cell phone just hours before the July 7 attacks.

Aswat, 31, was a British citizen of Indian origin and may not be in Pakistan, according to two intelligence officials in Islamabad and one in Lahore.

Both the Times of London and the Guardian reported that Aswat had been captured by Pakistani authorities while carrying guns, explosives and a large amount of cash and heading for the Afghan border.

The Times said he was linked to James Ujaama (search), an American convert to Islam accused of planning to set up an Islamist terrorist training camp in Oregon.

Ujaama, who while living in London ran the Web site of blind, hook-handed cleric Abu Hamza al-Masri (search), who preached to both "shoe bomber" Richard Reid (search) and Sept. 11 defendant Zacarias Moussaoui (search), pleaded guilty last year to aiding the Taliban. He is now a cooperating witness with American authorities.

FOX News' Wendell Goler, Catherine Herridge, Amy Kellogg and The Associated Press contributed to this report.

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