LONDON (Reuters) - Four blasts tore through packed underground trains and a bus during London's rush hour on Thursday, killing at least 37 people in the capital's deadliest peacetime attack and disrupting a summit of world leaders .

Police put the death toll at 37 by mid-afternoon, and French Interior Minister Nicolas Sarkozy later quoted Britain's government as saying 50 people had been killed.

Around 700 people were wounded, markets plummeted before partially recovering and Prime Minister Tony Blair rushed to London from the Group of Eight summit in Scotland after branding the attacks "barbaric." He was due to return later.

British Foreign Secretary Jack Straw said the attacks bore all the hallmarks of the Islamic militant al Qaeda network responsible for the Sept. 11, 2001, attacks on the United States and the Madrid train bombings in 2004.

Three explosions caused carnage on underground trains as Londoners made their way to work. The top was also ripped off a double-decker bus near Russell Square in the heart of the city.

"The scene afterwards was horrible: pieces of body on the ground," said Ayobami Bello, a 42-year-old security guard who was near the bus when it was torn apart. "If there are any survivors they will have very serious injuries."

Police said seven people were killed on an underground train near Liverpool Street, 21 were confirmed dead in another near King's Cross and seven died at Edgware Road. At least two passengers on the bus were killed.

Underground passengers stumbled through smoke-filled carriages to escape the trains after the blasts.

"It was horrific. There was smoke everywhere. I couldn't breathe," said Joe Witalls at Edgware Road station.

A previously unknown group, "Secret Group of al Qaeda's Jihad in Europe," claimed responsibility, but police said it was too early to say whether suicide bombers were involved.

"We are clearly shocked but we are not surprised by what has happened," assistant deputy commissioner of London police Brian Pad**** told reporters, adding no warning had been received.

President Bush, speaking at the G8 summit, told reporters: "The war on terror goes on."

"We will not yield to these terrorists, we will find them, we will bring them to justice," he said.

Britain is a key ally of the United States in Iraq, where al Qaeda is waging a bloody insurgency.

"It has the hallmarks of an al Qaeda-related attack. The assessment is currently being made," Straw said in a round of television interviews from the G8 summit in Scotland.

The blasts left Londoners angry and shocked. The Islamic Human Rights Commission warned London Muslims to stay at home to avoid any violence aimed at them.

OLYMPIC BLOW

The bombings came just a day after a jubilant London was awarded the 2012 Olympic Games.

"I'm deeply saddened that this should happen at the heart of an Olympic city. Unfortunately there is no safe haven. No one can say their city is safe," International Olympic Committee president Jacques Rogge said in Singapore.

Italy's interior minister said all Europe was on alert.

The carnage began shortly before 9 a.m. (0800 GMT) with the first blast and ended an hour and three more explosions later.

People were seen streaming out of underground stations covered in blood and soot. Hundreds of passengers were evacuated from stations across the capital, many in shock and with their clothes ripped to shreds, witnesses said.

London Mayor Ken Livingstone, speaking in Singapore, suggested suicide bombers may have been involved.

"I wish to speak to you directly -- to those who came to London today to take lives," he said. "I know that you personally do not fear to give your own life in exchange for taking others, which is why you are so dangerous."

Emergency services were pushed to the limit, with medics treating the wounded on train platforms and in a makeshift field hospital set up in a retail store. People with non-urgent complaints were urged to stay away from hospitals.

"We would put al Qaeda at the top of the list of any suspects," said Alex Standish, editor of Jane's Intelligence Digest. "They have the motives, the means and the opportunity.

"Here you have a massive headline event which involves the three world leaders that al Qaeda detests -- Bush, Blair and (Russian President) Putin," he said, speaking of the summit.

Oil prices fell over five percent before recovering and London stocks closed around two percent lower. Sterling sank to a 19-month low against the dollar and stayed there.

The city's streets rapidly emptied and financial markets initially fell sharply as it became apparent that the blasts were an attack rather than a power surge on the underground train system as had first been reported.

At dusk, hundreds of thousands of Londoners began a long walk home, with the underground network that carries 3 million passengers a day closed at least until Friday.

Thomas Carr, a 63-year-old electrician who faced a two-hour walk home, said he would keep using the underground.

"It won't put me off using the Tube," he said. "You can't let them beat you."