LONDON - A major terrorist attack has crippled central London killing at least 33 people and injuring hundreds of people on buses and in underground stations.

Brian Paddick, deputy assistant commissioner of the Metropolitan Police, said seven people were killed on an underground train near Moorgate station, 21 confirmed dead near Liverpool Street subway station and five dead at Edgware Road.

In a press conference ambulance staff said 45 people were in a serious or critical condition with burns, amputations, chest and blast injuries and broken limbs.

Another 300 had minor injuries with lacerations, smoke inhalation, shock and cuts and bruises.

People were also killed when a blast blew the roof off a double-decker bus near Russell Square but the number was not immediately clear.

The co-ordinated series of explosions rocked the city's transport system at rush hour on Thursday morning, causing scores of deaths and up to 1000 casualties and leaving victims and survivors fleeing in shock.  The first explosion was reported at 8.59am London time (7.59pm NZT).

Early today, Sky news was reporting that reliable sources said at least 45 people had died and a thousand injured, with fatalities likely to rise.

The city was paralysed after the entire underground network was shut down after blasts were reported at Aldgate East, Edgware Road, Old Street, King's Cross, Russell Square and Moorgate and Liverpool Street.

Elaph, a secular Arabic-language news website, and Germany's Der Spiegel news magazine reported a group, "Secret Organisation - al Qaeda in Europe" - has claimed responsibility in retaliation for Britain's involvement in Iraq and Afghanistan.

London Mayor Ken Livingstone said the blasts that ripped through his city were "mass murder" carried out by terrorists bent on "indiscriminate ... slaughter."

"That isn't an ideology, it isn't even a perverted faith, it's mass murder," Livingstone said. "We know what the objective is. They seek to divide London."

He was rushing home from Singapore, where he spearheaded London's winning bid to host the 2012 Olympics, following the news.

The bombings, with their echoes of the deadly Madrid bombing which killed 191 people last year, sent shockwaves across Europe and North America, with security forces stepping up alerts and increasing patrols on mass transport, while sending stockmarkets into decline.

Paul Woodrow, an official with the ambulance service, told reporters that rescue operations were ongoing and that "there are large numbers of casualties and we believe there are a number of fatalities".

At least seven people died when a bus was also ripped apart in an explosion in Russell Square - a suicide bomber was being linked to that explosion, which some media reported occurred on a bus packed with people forced off the underground when the network was shut down.