The four successive blasts tore into three London Underground trains and one double-decker bus.
The worst attack on London since the second world war came early on Thursday, just a day after an overjoyed city celebrated its successful bid to hold the 2012 Olympics.
Twenty-one people died at the King's Cross station, seven at Edgware Road, seven at Liverpool Street and two on the bus near Russell Square - a total of 37, Metropolitan Police said. Another 700 people were injured, they added. Police and transport officials said the underground railway network would be seriously affected for some time.
Deputy Assistant Commissioner Brian Paddick said there had been no arrests, and said it was unclear whether suicide bombers were involved.
He said police were examining a claim of responsibility on the internet purportedly on behalf of an organisation linked to al-Qaida, but could not confirm its authenticity.
Passengers were evacuated from
many underground train stations
Security experts said the incidents bore all the hallmarks of an al-Qaida attack.
"If what we are looking at is a simultaneous bombing - and it does look like that - it would very certainly fit the classic al-Qaida methodology," said Shane Brighton, intelligence expert at the Royal United Services Institute for Defence.
Paddick said the first explosion, at 8.51am (0751 GMT), caught a Circle line train that was 100m into a tunnel outside Moorgate station in the financial district in east London. Seven died, he said.
The second blast, at 8.56am, hit a Piccadilly line train between the King's Cross and Russell Square stations, killing 21.
The third explosion, at 9.17am, killed seven people, and involved two and possibly three trains, Paddick said.
The blast that ripped apart the double-decker bus near Russell Square was reported at 9.47am. Two were killed.
British Prime Minister Tony Blair described the blasts as an apparent terrorist attack coinciding with a meeting of Group of Eight leaders in Scotland.
"It is reasonably clear that there have been a series of terrorist attacks in London," Blair said at the summit. He later returned to London for a few hours.
Transit shut down
The entire underground system was shut down and major thoroughfares were blocked off by police and ambulance services.
London's police chief Ian Blair said there were indications of explosives at one of the blast sites.
Motorway traffic was warned to
avoid London after the blasts
"We are aware that one of the sites certainly does contain indications of explosives," he told Sky Television. "We are concerned that this is a coordinated attack."
Financial markets took fright, with stocks diving and demand for government bonds and safe-haven currencies soaring.
Transport officials, appearing at a briefing with Paddick, said limited Tube and bus service would resume on Thursday night.
Covered in blood
People were seen streaming out of one underground station covered with blood and soot. Passengers were evacuated from stations across the capital, many in shock and with their clothes ripped to shreds, witnesses said.
A doctor at Aldgate underground station in the east of the financial centre of the city said at least 90 people were wounded at that location.
Ambulances at the ready in front
of London's Kings Cross Station
Police sealed off large areas around other underground and mainline railway stations. Firefighters donned chemical protection suits before rushing into stations.
Initially, rail officials blamed the explosions on a power surge.