A shaken British Prime Minister Tony Blair, hosting the G-8 leaders in Scotland, declared London had suffered "a series of terrorist attacks" and called it "barbaric."
"There are people who have died and people who are injured," he said, adding that it was clear the attacks had been carried out to coincide with the summit.
The explosions caused officials to shut down the entire bus and underground transport network. They came a day after London was awarded the 2012 Olympics.
Sky News showed a picture of a mangled red bus with its top collapsed. Bloodied and bandaged witnesses reported panicked crowds fleeing as the blast rained glass and other debris onto the street.
BBC TV broadcast footage of a paramedic trying to revive one of the wounded, pumping the chest of his bloodied and blackened body.
"There have been a number of dreadful incidents across London today," said Home Secretary Charles Clarke, Britain’s top law enforcement officer. He said there were "terrible injuries."
Germany’s Der Spiegel news magazine reported a group calling itself "Secret Organization — al-Qaeda in Europe" had posted a claim of responsibility for the blasts on a website popular with Islamic militants, which it did not name. It said the group claimed the explosives were in retaliation for Britain’s involvement in Iraq and Afghanistan.
Police said they had reports of at least two deaths and nine people injured in six blasts. 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." Officials at the Royal London Hospital told BBC that 95 injured had been brought into that hospital alone.
London police chief Ian Blair said authorities found indications of explosives at the scene of one of the blasts. He said there had been no warning.
"We are concerned that this is a coordinated attack. We are aware that one of the sites certainly does contain indications of explosives," Blair said on Sky News.
"We have been at a very high state of alert. Of course, if there had been any kind of specific warnings we would have dealt with it," he said.
The explosions sent stocks plummeting in Europe. The FTSF 100 dropped 207.54 points to 5022.10 by early afternoon. The DAX was down 3 percent at 4.474.35, while in Paris, the CAC 40 dropped 3.5 percent. Exchange elsewhere in Europe was also down. — Pia Lee-Brago