Emergency response: The Underground remained closed last night, with many bus services suspended, after the deadliest ever peacetime attack on Britain killed at least 37 people and injured 700.
Police warned the public to be vigilant in case of further attacks, adding: "It may not be over."
The four attacks within 51 minutes yesterday morning brought London's public transport system to a halt, filled all of its major hospitals and overwhelmed the city's mobile phone network.
The first bomb went off at 8.51am on a Metropolitan line Underground train between Liverpool Street and Aldgate, in the heart of London's business district. The blast, which killed seven people, was at first attributed to an electrical power surge.
Five minutes later, a second explosion blew apart a Piccadilly line train between Russell Square and King's Cross, killing 21 people.
At 9.17am, a bomb exploded on a Circle line train leaving Edgware Road station for Paddington, blasting through a wall to hit at least one other train on a platform.
After the three Underground explosions, London Transport chief Peter Hendry ordered buses to be opened to all passengers.
"Then the police told us to stop the buses," he said.
The police order came after a number 30 double-decker bus near Tavistock Place in Bloomsbury was rocked at 9.47am by a massive explosion that tore its roof off and damaged nearby cars.
Police said last night that they did not know if a suicide bomber had caused the bus explosion or if it had been caused by a bomb planted earlier.
London's Air Ambulance was scrambled within minutes of the first attack, dropping medical staff to the scene to treat the most severely wounded.
Russell Smith, of London Ambulance Service, said that over 100 ambulances with 250 staff were sent to the four bomb scenes. "They treated 45 patients for serious burns, amputations, chest and blast injuries and fractured limbs."
A further 300 people were treated at the bomb scenes for more minor injuries such as lacerations, shock, cuts and bruises, and hundreds more were taken to hospitals around central London.
Doctors said some of the injuries were likely to cause lifetime disability or years of treatment, including skin grafts for burn victims.
As passengers were led to safety from the bombed trains and bus by police and emergency staff, mobile phones stopped working, leaving them unable to contact their relatives.
Police and phone companies dismissed reports that mobile phone networks had been blocked in response to the attacks, explaining that the system was unable to cope with the sudden surge in calls.
With all public transport suspended, police asked Londoners to remain at work or at home and to avoid all but the most essential journeys. River boats carried passengers free of charge from the Docklands into central London, and taxis were surrounded each time they set down passengers.
London's deputy police commissioner Brian Paddick said police had received no warning and no admission of responsibility for the attacks.
"The police service received no warning about these attacks, and the police service has not received any claims of responsibility from any groups in connection with these attacks. This clearly was a callous attack on purely innocent members of the public, deliberately designed to kill and injure innocent members of the public."
Police declined to comment on a statement published on an Islamist website yesterday in the name of a previously unknown group, the Secret Organisation Group of al-Qaeda of Jihad Organisation in Europe.
"The heroic mujahidin have carried out a blessed raid in London. Britain is now burning with fear, terror and panic in its northern, southern, eastern and western quarters. . .O nation of Islam and nation of Arabism: rejoice for it is time to take revenge from the British Zionist Crusader Government in retaliation for the massacres Britain is committing in Iraq and Afghanistan," it said.
Police sources said the attacks bore all the hallmarks of al-Qaeda attacks on Bali, Istanbul, Madrid and Casablanca - co-ordinated bombings targeting civilians without warning.
As police and security services faced charges of an intelligence failure, transport police warned that London's public transport system would remain disrupted today.
For millions of Londoners last night, the biggest challenge was finding their way home amid the traffic chaos caused by the worst attack on their city since the second World War.