More than 50 people were killed in yesterday's bomb attacks in London, Metropolitan Police Commissioner Sir Ian Blair said today.
A Tube passenger this morning.
He said there was "great difficulty" in determining the number of fatalities because of the damage at scenes of the bombs blasts on the rush-hour Tube system and a bus.
Andy Hayman of the city's specialist operations branch said police had yet to reach one of the London underground train carriages where a bomb went off at rush hour yesterday.
"When I describe the scene in Russell Square, it has yet to be the case for us to get near the carriage," he told a press conference. "There is the threat of the tunnel being unsafe."
The police said no survivors were trapped underground and the task now was to retrieve bodies. One police source said there could be more than 10 bodies still underground.
A maintenance worker, who did not want to be identified, said he had reached the site early today and described "awful" scenes, with several bodies in the carriage.
"We got up to the carriage, although it was very dark there at the time," he said. "The smell was awful."
The blast on the underground train between King's Cross and Russell Square stations killed at least 21 people.
"The complexity of getting to the carriage is one of safety," Mr Hayman said. "Just imagine an explosion that far into a tunnel in that number of the carriage, I think we can all respect the sort of things our people are actually confronting," he added.
"I think out of respect for others that's probably the most detail I'd want to go into."
Speaking at the same press conference, Sir Ian revealed there were 700 casualties, 350 were taken to hospital, 22 were in a critical condition and one person died in hospital. The commissioner said that the casualty bureau had already received 104,000 calls.
He said there was "absolutely nothing to suggest that this was a suicide bombing attack although nothing at this stage can be ruled out".
He agreed that the attack "bore all the hallmarks" of an al-Qaeda operation. Anti-terrorist branch head Andy Hayman said each of the bombs contained less than 10lbs of high explosives and they were probably placed on the floor of the three Tube trains or, in the case of the bus, on the floor or a seat.
A major investigation is under way this morning to find the perpetrators of the attack. The area around Russell Square where the roof of a double-decker bus was ripped off by a bomb remains cordoned off as police continue their forensic investigations.
The three bombed underground trains were also being treated as crime scenes, which has meant engineers have been unable to assess the damage.
Commuters returned to work on a transport system that was hit with four bombs during yesterday's rush hour. Euston Station was evacuated for a time this morning after a security alert.
Transport for London said buses were running a nearly full service and that much of the Tube was also operating.
"There will be some delays and parts of the tube will not be open," a transport spokesman said. "We are aiming for a near normal service."
The underground rail network, one of the world's largest, usually carries three million passengers a day, and the Metropolitan Police asked commuters to consider whether it was essential they go to work.
The Circle, Hammersmith and City Tube lines are to remain closed. Buses will operate normally, except near the immediate areas where the explosions happened.
British prime minister Tony Blair has returned to Gleneagles, Scotland, where he is hosting the leaders of the world's eight richest nations. The G8 summit will press ahead with its discussions on climate change and poverty.
A group that claims to have links with al-Qaeda said in an Internet statement earlier that it had carried out the attacks. The unverified claim, made on the Al-Qal'ah - Fortress - Internet site said that "Britain is burning with fear".
An unverifiable claim of responsibility was posted on an al-Qaeda linked website. "Britain is now burning with fear, terror and panic," a message claimed.
The first explosion happened at 8.51am in the Moorgate, Liverpool Street and Aldgate East underground stations. The second explosion happened at 8.56am in the King's Cross, Russell Square area, leaving at least 21 people dead.
The third was at 9.17am when an explosion hit Edgware Road Underground station. The blast blew a hole in a train on an adjoining platform.
The fourth explosion happened at 9.47am on a bus in Tavistock Place. There have been two confirmed fatalities in the bus attack. The involvement of a suicide bomber has not been ruled out in this attack. No warnings were given before any of the attacks.