At least 33 people have been killed in a series of four separate explosions in London, the city's police says.

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.

Seven were killed in the first explosion in an underground railway tunnel near Moorgate on the edge of London's financial district, 21 in a second near King's Cross and another five at Edgware Road station in west London.

An unknown number of people also died in a blast on a bus, Deputy Assistant Commissioner of Police Brian Paddick said.

An ambulance service official said that 345 people were injured, 45 of them seriously.

Police and transport officials said the underground rail network would be seriously affected for some time.

Paddick said there had been no arrests, and said it was unclear whether suicide bombers were involved.

Claim

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.

Transport officials, appearing at a briefing with Paddick, said limited Tube and bus service would resume Thursday night.

Paddick said the first explosion, at 8.51am (0751GMT), caught a Circle line train that was 100 yards (meters) into a tunnel outside Moorgate station in the financial district in east London. Seven died, he said.

The second blast, at 8.56am (0756GMT), hit a Piccadilly line train between the King's Cross and Russell Square stations, killing 21, Paddick said.

The third explosion, at 9.17am (0817GMT), killed five 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 (0847 GMT).

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 added that he would return to London in the "next couple of 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.

"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."

There was no immediate claim of responsibility.

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.

Initially, rail officials blamed the explosions on a power surge.

Hallmarks

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.

Financial markets took fright, with stocks diving and demand for government bonds and safe-haven currencies soaring.

The first reports of blasts centred on the city's underground railway system.

Aldgate East station

Emergency services rushed to Aldgate East at 8.59am (0759 GMT).

"There were people streaming out of Aldgate station covered in blood," said witness Kate Heywood, 27, on her way to work.

A Reuters correspondent at Oxford Circus, at the heart of the system, heard an announcement over the public-address system saying: "A power outage has occurred London-wide. All train services are suspended."

Police sealed off large areas around other underground and mainline railway stations. Firefighters donned chemical protection suits before rushing into stations.

Half a dozen people with soot-blackened faces and dishevelled clothes sat on the floor at Russell Square station or stood in shock as police cordoned off the area and ambulance crews raced in, one witness said.

PHOTO CAPTION

An emergency worker walks near the scene of an explosion near Russell Square in London July 7, 2005. (Reuters)