LONDON -- An eyewitness to the bombings in London says he'll be haunted by the image of someone "whose face was totally black and pouring with blood."

Another said a subway car "was completely gutted" by one of the four blasts that hit Thursday morning.

British police said the official confirmed death count so far in the attacks on London's transit system has risen by four to 37, with 700 others injured. A U.S. law enforcement official said at least 40 people were killed. The official spoke on condition of anonymity, but said U.S. authorities learned of the number from their British counterparts.

Whatever the final death toll, it's the worst attack in London since World War II.

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Related: Terror Alert Rises For U.S. Transit Systems
Map: Locations Of London Bomb Blasts
Fact Sheets: Attacks Since 9/11 | Britain Terror Attacks | Decade Of Subway Attacks

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Morning rush hour in London turned into a scene of carnage Thursday morning, and experts say the attacks look like al-Qaida's handiwork.

Belinda Seabrook was riding a bus to work in London when she heard what she called "an incredible bang." The double-decker bus behind hers had blown up -- one of four coordinated rush-hour explosions.

"I turned round," Seabrook said, "and half the double-decker bus was in the air."

Jay Kumar, who saw the explosion from his business nearby, said he saw "debris flying all over, mostly glass."

Doctors from the British Medical Association building rushed out to treat the wounded. BMA House was splattered in blood "and not much of the bus was left," said Dr. Lawrence Buckman.

On the subway, commuter Simon Corvett said he heard "this massive, huge bang." "All the windows shattered," Corvett said. He said the subway car across from his "was completely gutted."

It looks like the work of al-Qaida, according to British Foreign Secretary Jack Straw. He said the bombings have the "hallmarks" of an al-Qaida attack.

Straw said British police and the country's intelligence agencies hadn't received any warnings ahead of time.

A security expert at St. Andrews University in Scotland agreed that the series of explosions have "all the trademarks of the al-Qaida network." He said the blasts on subways and a bus were "clearly aimed to cause casualties among the public."

Another expert at the university said London and its transportation network are "a very tempting target." Magnus Ranstorp added, "It's impossible to guard against this."

He said at least 10 to 20 people would have been involved in Thursday's bombings. He said any speculation of a connection to London being picked for the 2012 Olympics is "nonsense," because such an attack requires a lot of planning. Ranstorp said the bombings were likely intended to disrupt the Group of Eight talks going on in Scotland. (Click here for full G-8 story.)

Deputy Assistant Commissioner Brian Paddick told reporters that there were four 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.

He said officials aren't sure yet if suicide bombers or package bombs were used in what he called "a callous attack."

An official with the London Ambulance Service said at least 345 people were injured. He said at least 45 of those were in serious or critical condition. He said the most serious injuries include "amputations, burns" and other wounds.

The entire transportation system was shut down. A police official said the city is trying to get buses rolling again, but there have been "a number of bomb scares." A transit official said all buses are being "searched regularly."

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 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."

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.

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

British Prime Minister Tony Blair vowed that despite the attacks, terrorists wouldn't succeed.

"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," 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.

Officials Try To Restore Transportation

London officials said they're working to get the bus and rail system back rolling after today's attacks.

A police official is urging city residents to start "thinking about their journey home." He said it would be "a difficult evening" for commuters.

The police official said the underground train system "will be out of action" until Friday. But the city hopes to get other means of travel moving before that.

A transit official said it could take some time for buses to return. He said they will be "searched regularly."

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.