The explosions in London have all the trademarks of Al Qaeda (search): Attacks on crowded trains and other hard-to-guard public places, an effort to wreak maximum chaos and a quick claim of responsibility.

The goal appears the same, also: To pressure countries with troops in Iraq to pull out.

Thursday's attack in London strongly resembled the string of bombings on four commuter trains in Madrid, Spain, that killed 191 people in March 2004. In that case, the bombs were placed in knapsacks on the trains.

In the London attacks, which killed dozens and wounded hundreds, there was no immediate sign that homicide attackers set off the blasts.

"This one is very similar to the attack in Madrid: a soft target, full of people, coordinated attacks, at rush hour. All are indications of Al Qaeda," said Diaa Rashwan, a Cairo-based expert on militants.

But the London bombings may have even greater significance, he believes, because of the British capital's international status and because of the timing. The blasts on three underground trains and a double-decker bus came as leaders of the Group of Eight (search), the world's most powerful nations, were meeting in Scotland.

"There is ... a very high alert for the G-8 summit. For such an attack to take place despite all of that is indisputable proof of the failure of American and British security policies," said Rashwan, of the Al-Ahram Center for Strategic Studies (search).

The bombings underline the difficulty of defending public facilities in Europe, said Magnus Ranstorp, a terror expert at St. Andrews University in Scotland.

London and its transportation network are "a very tempting target; it's one of the most obvious targets in Europe," he said. "It's impossible to guard against this."

A single group, Al Qaeda in Europe (search), claimed both the Madrid and London bombings. But that may just be a name covering a number of cells working in coordination, some experts believe.

Ten to 20 people in several loosely affiliated groups were likely involved in the London attacks because mounting such synchronized attacks "requires major coordination," Ranstorp said.

British Foreign Secretary Jack Straw said the explosions have the "hallmarks of an Al Qaeda related attack.

Mohammed Salah, an expert on Islamic radicals and the Cairo bureau chief of the Arab newspaper Al-Hayat, said the operation may have been carried out by a "sleeper cell" overlooked by British security. "Something similar to the Hamburg cell led by Mohammed Atta," he said, referring to one of the Sept. 11 hijackers, who had been based in Germany.

Usama bin Laden and his deputy in Al Qaeda, Ayman al-Zawahri, have threatened Britain along with other countries in their recent messages, he noted.

"It proves that they still exist and at a large scale despite the war against terrorism," he said of the London bombings.

After the Madrid attacks, Spain tried to dismantle the cell responsible — and ended up uncovering other cells planning different attacks. Spanish authorities have arrested more than 130 suspected extremists in the past year.

The Spanish investigations picked up trails of cells in Italy, France and other countries across Europe.

Another link between the Madrid and London bombings is Iraq — although Spanish authorities have questioned whether Spain's military presence in Iraq was really a motivation for the Madrid bombings as claimed by Al Qaeda in Iraq.

On Thursday, a statement on an Islamic Web site signed by the group's "secret organization" said Thursday's bombings were a "response to the massacres Britain committed in Iraq and Afghanistan."

It also warned Denmark and Italy to expect "the same punishment if they do not withdraw their troops from Iraq and Afghanistan."

Spain pulled its 1,300 troops out of Iraq after Prime Minister Jose Maria Aznar's conservative government was defeated by the Socialists in elections March 14, three days after the Madrid bombings.