British interior minister Charles Clarke called the explosions "terrorist attacks."
Witnesses saw the top ripped off a double-decker bus near Russell Square close to King's Cross train terminal, and three more apparently coordinated explosions caused carnage on packed subway trains as Londoners made their way to work.
Security sources told Reuters there were fatalities at all four bomb sites, and Sky Television said at least 45 people had been killed. A further 150 were seriously wounded, and hospital staff said some were unlikely to survive.
"I was on the bus," said one dazed passenger. "I looked 'round and the seats behind me were gone."
"You could see bodies on the road outside," said another eyewitness, Peter Gordon. "There was smoke everywhere. It was carnage."
President Bush, speaking at the G-8 summit, told reporters that "the war on terror goes on."
"We will not yield to these terrorists, we will find them, we will bring them to justice," he said.
A previously unknown group, Secret Group of al-Qaida's Jihad in Europe, claimed responsibility for the attacks.
Police have so far declined to give a death toll or comment on suggestions that suicide bombers were involved.
Financial markets tumbled as the scale of the attacks became clear, and Blair told reporters he would return to London from the G-8 summit to oversee the emergency. He planned to return to the talks in Gleneagles later in the day.
The attacks recalled the 2004 train bombings in Madrid, which were blamed on al-Qaida, and left Londoners in shock. The Islamic Human Rights Commission warned London Muslims to stay at home to avoid any violence aimed at them.
The attacks came a day after a jubilant London was awarded the 2012 Olympic Games.
"I'm deeply saddened that this should happen at the heart of an Olympic city. Unfortunately, there is no safe haven. No one can say their city is safe," said International Olympic Committee president Jacques Rogge in Singapore.
Italy's interior minister said all Europe was on alert.
Britain has been key ally of the United States in its war in Iraq, where al-Qaida is waging a bloody insurgency. The blasts occurred one day after London was awarded the 2012 Olympics.
Police said two people were killed at Aldgate East underground station in the financial center of the city, with a further 90 people wounded. Around 100 wounded people were taken to Royal London Hospital, 10 of them in critical condition.
London's police chief Ian Blair said there were indications of explosives at the blast sites.
"We are concerned that this is a coordinated attack," he told Sky television.
London Mayor Ken Livingstone, speaking in Singapore, said suicide bombers may have been involved.
"I wish to speak to you directly--to those who came to London today to take lives," he said. "I know that you personally do not fear to give your own life in exchange for taking others, which is why you are so dangerous."
People were seen streaming out of one underground station covered with blood and soot. Passengers were evacuated from stations across the capital, many in shock and with their clothes ripped to shreds, witnesses said.
The city's streets rapidly emptied and financial markets plummeted as it became increasingly apparent that the blasts were an attack, and not a power surge on the underground train system as had first been reported.
In the wake of the blasts,as people attempted to contact friends and relatives through other means.
"As usual, the computer seems to be the best way to communicate," said Nick Sheth, a London-based business development manager for Yahoo. Sheth was using both e-mail and instant messaging to let people know he was OK.
Security experts said the blasts bore all the hallmarks of al-Qaida.
"If what we are looking at is a simultaneous bombing, and it does look like that, it would very certainly fit the classic al-Qaida methodology which centers precisely on that: multi-seated hits on transport and infrastructural targets," said Shane Brighton, intelligence expert at the Royal United Services Institute for Defense.
On the currency market, the safe-haven Swiss franc hit a six-week high against sterling and rose more than 1 percent against the dollar following the explosions.
"The market is showing a textbook reaction, buying safe-haven currencies like the Swiss franc and euro and away from the dollar," said Marios Maratheftis, currency strategist at Standard Chartered.
Oil prices initially fell 3 percent before recovering and London's FTSE stock exchange lost 2 percent.
CNET News.com's Ina Fried contributed to this report.