LONDON - Four explosions rocked the London subway and tore open a packed double-decker bus during the morning rush hour Thursday, sending bloodied victims fleeing in the worst attack on London since World War II. At least 40 people were killed, U.S. officials said, and more than 700 were wounded.

A clearly shaken Prime Minister Tony Blair called the coordinated attacks "barbaric" and said they were designed to coincide with the G-8 summit opening in Gleneagles, Scotland. They also came a day after London won the bid to host the 2012 Olympics. A group calling itself the Secret Organization of al-Qaida in Europe claimed responsibility.

British Foreign Secretary Jack Straw said the explosions have the "hallmarks of an al-Qaida related attack. He added that neither Britain's police nor the intelligence services had any warning of the attacks.
The four blasts went off within an hour, beginning at 8:51 a.m. (3:51 a.m. EDT), and hit three subway stations and the double-decker bus. Authorities immediately shut down the subway and bus lines that log 8.4 million passenger trips every weekday.
The bus explosion seemed to go off at the back of the vehicle, said bystander Raj Mattoo, 35. "The roof flew off and went up about 10 meters (30 feet). It then floated back down," he said. "There were obviously people badly injured. A parking attendant said he thought a piece of human flesh had landed on his arm."
Doctors from the nearby British Medical Association rushed into the street to treat the wounded from the bus. "The front of BMA house was completely splattered with blood and not much of the bus was left," said Dr. Laurence Buckman.
"It was chaos," said Gary Lewis, 32, evacuated from a subway train at King's Cross station. "The one haunting image was someone whose face was totally black and pouring with blood."
As the city's transportation system ground to a near-halt, buses were used as ambulances and an emergency medical station was set up at a hotel. Rescue workers, police and ordinary citizens streamed into the streets to help.
Some central London streets emptied of traffic. Groups of commuters who had been on their way to work gathered around corner shops with televisions, watching in silence. The mood was somber and subdued.
At the request of Queen Elizabeth II, the Union Jack flag flying over Buckingham Palace was lowered to half staff.
Blair, flanked by fellow G-8 leaders, including President Bush, read a statement from the leaders. "We shall prevail and they shall not," he 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," he said earlier.
Returning to London, he promised an intense police hunt for those responsible. He said he knew most Muslims worldwide "deplore this act of terrorism."
"They are trying to use the slaughter of innocent people to cow us," he said. "They should know they will not succeed."
In Scotland, police evacuated a section of Edinburgh's main shopping street after a suspect package was found on a bus.
In Scotland, Bush warned Americans to be "extra vigilant," and his administration raised the terror alert for mass transit a notch to code orange. Security also was stepped up in the U.S. Capitol and in train and bus stations around the country.
Much of Europe also went on alert. Italy's airports raised alert levels to a maximum. The Czech Republic, Hungary, Russia, the Netherlands, France and Spain also announced beefed-up security at shopping centers, airports, railways and subways.
The U.N. Security Council was expected to pass a resolution condemning the blasts later Thursday, an official said.
A group calling itself "The Secret Organization of al-Qaida in Europe" posted a claim of responsibility, saying the blasts were in retaliation for Britain's involvement in Iraq and Afghanistan.
The statement also threatened attacks in Italy and Denmark, both of which have troops in Iraq. It was published on a Web site popular with Islamic militants, and the text was republished on Elaph, a secular Arabic-language news Web site, and Berlin's Der Spiegel magazine.
The statement's authenticity could not be immediately confirmed, but terrorism experts said the blasts had the trademarks of the al-Qaida network.
"This is clearly an al-Qaida style attack. It was well-coordinated, it was timed for a political event and it was a multiple attack on a transportation system at rush hour," said Lawrence Freedman, professor of war studies at King's College in London.
Deputy Assistant Commissioner Brian Paddick said there had been no arrests, and it was unclear whether suicide bombers were involved.
Asked about the claim of responsibility, Paddick said: "We will be looking at that ... at the moment we don't know if that's a legitimate claim or not." He added British officials had received no prior warning or advance intelligence that the attacks would occur.
European stocks dropped sharply after the blasts, with exchanges in London, Paris and Germany all down about 2 percent. Insurance and travel-related stocks were hit hard, and the British pound also fell. Gold, traditionally seen as a safe haven, rose.
The explosions also unnerved traders on Wall Street, sending stocks down sharply.
Three U.S. law enforcement officials said at least 40 people were killed. They spoke on condition of anonymity and said they learned of the number from their British counterparts.
In London, police said they could confirm at least 37 people had been killed, including two in the bus attack.
Buckman, the London doctor, said ambulance staff told him about 10 people died in the bus blast. BMA doctors treated about nine seriously wounded people in the building's courtyard, two of whom later died, he said.
London police said at least 700 were wounded. Among them, at least 45 were in serious or critical condition, including amputations, fractures and burns, hospital officials told The Associated Press.
London Mayor Ken Livingstone said the blasts were "mass murder" carried out by terrorists bent on "indiscriminate ... slaughter."
"This was not a terrorist attack against the mighty or the powerful ... it was aimed at ordinary working-class Londoners," said Livingstone, in Singapore where he supported London's Olympic bid. Giselle Davies, an International Olympic Committee spokeswoman, said the committee still had "full confidence" in London.
Jay Kumar, a business owner near the site of the bus blast, said he ran out of his shop when he heard a loud explosion. The bus's top deck collapsed, sending people tumbling to the floor, he said. Bloodied people ran from the scene.
"A big blast, a big bomb," he told The Associated Press. "People were running this way panicked. They knew it was a bomb. Debris flying all over, mostly glass."
"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.
Traces of explosives were found at two explosion sites, a senior police official said.
Explosions were reported at the Aldgate station near the Liverpool Street railway terminal, Edgware Road and King's Cross in north London, Old Street in the financial district and Russell Square, near the British Museum.
"I saw lots of people coming out covered in blood and soot. Black smoke was coming from the station. I saw several people laid out on sheets," office worker Kibir Chibber, 24, said at the Aldgate subway station.
Simon Corvett, 26, on an eastbound train from Edgware Road station, described "this massive huge bang ... It was absolutely deafening and all the windows shattered."
"You could see the carriage opposite was completely gutted," he said. "There were some people in real trouble."
On March 11, 2004, terrorist bombs on four commuter trains in Madrid killed 191 people.
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