LONDON -- Morning rush hour in London turned into a scene of carnage Thursday morning.

Explosions blamed on terrorists targeted the capital city's mass transit system, hitting subways and ripping apart a double-decker bus.

London authorities confirm more than 33 fatalities in today's bomb attacks.

A police official told reporters that there were four separate bomb attacks -- including one at a subway station that may have affected three separate trains. He confirmed 33 fatalities in the subway attacks -- and said there were additional fatalities in an attack on a double-decker bus. But no numbers from that attack are being released.

City hospitals report they are treating more than 300 people for injuries from the four explosions. A senior police official put the number of serious injuries at about 150.

Police said traces of explosives have been found at two of the blast sites.

The explosions sent panicked, bloodied and blackened passengers and witnesses fleeing debris-strewn blast sites. One witness said there was debris everywhere, much of it splintered glass.

A U.S. law enforcement official said at least 40 people were killed in the explosions in London. The official spoke on condition of anonymity, because British officials haven't yet made public the death toll. The official said U.S. authorities learned of the number from their British counterparts.

A newsstand operator said he heard what sounded like a "big bomb" and ran out to see the top of a bus blown off and people tumbling down. He said debris flew all over as people ran in panic.

BBC television footage showed a paramedic trying to revive a victim, pumping on the chest of a bloodied and blackened body. A subway passenger said train cars filled with smoke and there were "loads of people screaming." He said, "You couldn't really breathe and you couldn't see what was happening."

Another said there was a loud bang and then a flash of flames down the side of his train.

The nearly simultaneous explosions came a day after London was awarded the 2012 Olympics and as the G-8 summit was getting under way in Scotland. (Click here for full G-8 story.)

"It's reasonably clear that there have been a series of terrorist attacks in London. There are obviously casualties, both people that have died and people seriously injured. My thoughts and prayers are with the victims and their families," British Prime Minister Tony Blair said.

Blair is hosting the world's most powerful leaders at Gleneagles, Scotland.

Blair left the G-8 to get a report from those who have been dealing with the attacks.

All the leaders decided that the meetings should continue in Blair's absence.

"Each of the countries around that table have some experience of the effects of terrorism and share our complete resolution to defeat this terrorism," Blair said. "It's particularly barbaric that this has happened on the day when people are meeting to try to help the problems of poverty in Africa and the long-term problems of changes in the environment."

"It's reasonably clear that it is designed and aimed to coincide with the opening of the G-8. There will be time to talk later about this. It's important, however, that those engaged in terrorism realize that our determination to defend our values and our way of life is greater than their determination to cause death and destruction to innocent people in a desire to impose extremism on the world," Blair said. "Whatever they do, it is our determination that they will never succeed in destroying what we hold dear in this country and in other civilized nations throughout the world."

A group calling itself "The Secret Organization of al-Qaida in Europe" posted a claim of responsibility for the blasts, saying they were in retaliation for Britain's involvement in Iraq and Afghanistan. The Web statement, republished on the site of the German magazine Der Spiegel, could not be immediately confirmed. The group al-Qaida in Europe claimed responsibility for the last major attack in Europe -- a string of bombs that hit commuter trains in Madrid, Spain, in March 2004, killing 191 people.

The first explosion was reported at 8:50 a.m. London time, 3:50 a.m. EDT.

People covered in blood stumbled out of the subway, and a witness said the entire top deck of the bus was destroyed.

"I was on the bus in front and heard an incredible bang, I turned round and half the double-decker bus was in the air," Belinda Seabrook told Press Association, the British news agency.

She said the bus was packed with people.

"It was a massive explosion and there were papers and half a bus flying through the air," she said.

Officials shut down the entire underground network and the entire bus system after the explosions. Initial reports blamed a power surge, but officials were not ruling out an intentional attack.

Witness William Chamberlain said there were multiple casualties and multiple bodies strewn about the scene of the bus explosion. He said the bus was "torn apart like a tin can."

One witness, Darren Hall, said some passengers emerging from an evacuated subway station had soot and blood on their faces. He told BBC TV that he was evacuated along with others near the major King's Cross station and only afterward heard a blast.

The entire cell phone system has crashed in London.

Meanwhile, in the United States, the Washington, D.C., Metro system said it stepped up security immediately after an explosion on the subway system in London Thursday morning. Metro spokeswoman Candace Smith said passengers should expect to see increased police patrols, bomb-sniffing dogs and officers carrying machine guns. There have not been any reports of direct threats to the subway system.

Bush Denounces Terrorists

President George W. Bush said those behind the blasts have "evil in their heart." And meanwhile, he's urging U.S. authorities to be "extra vigilant."

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Bush: Attackers 'Evil' At Heart

The president spoke to reporters during a break in the G-8 summit -- and after consulting with his homeland and national security chiefs by secure video link from Gleneagles, Scotland.

Bush said he goes with a "message of solidarity" from the other leaders. Bush said the contrast between a summit aimed at easing poverty and terrorists killing innocents is "incredibly vivid."

He said he was "most impressed" by the resolve of the summit leaders -- in his words -- not to "yield to these people."

Israel Was Not Warned

Israel's foreign minister said his country was not warned about possible terror attacks in London. Silvan Shalom said there was "no early information about terrorist attacks."

Earlier, a Foreign Ministry official said British police had warned the Israeli Embassy in London of possible terror attacks minutes before the first explosion.

Israel was holding an economic conference in a hotel over the subway stop where one of the blasts occurred.

Shalom said he's not aware of any Israeli casualties. He said the attackers might have taken advantage of the fact that police resources were diverted to a meeting of Western leaders.