A series of explosions have rocked London's transport system at rush hour, causing numerous casualties and leaving millions of passengers in shock, police say.

The causes of the Thursday morning incidents - which occurred one day after London was awarded the 2012 Olympics and as leaders of the Group of Eight rich nations met in Scotland - were unknown but police said many of the city's underground stations were affected.

"It is too too early to state what has happened," a London police spokesman said. "I cannot comment on reports of bombs, but we have had multi-reports of explosions around London."

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.

The entire system was shut down and major thoroughfares were blocked off by police and ambulance services.

Bus explosion

An explosion also occurred on a bus at Tavistock Square in central London, a police officer said.

"There have been some casualties and this has been declared as a major incident," said a spokeswoman for London's Metropolitan Police.

Emergency services rushed to the Aldgate East underground station, where police reported one incident at 8:59am (0759 GMT), the spokeswoman said.

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

"There are shards of glass there - it is chaos," she added.

Power outage reported

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

A station official said: "All the power has gone down. I don't know when it'll come back."

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 the Russell Square underground station or stood in shock as police cordoned off the area and ambulance crews raced in, one witness said.

PHOTO CAPTION

A frame grab shows emergency vehicles on the scene outside King's Cross station in London July 7, 2005. (Reuters)