Witnesses have told how an explosion tore the roof off a double-decker bus, killing at least two people, in one of the series of bomb attacks in London.
Witnesses said the roof of the bus was blown into the sky
One described seeing bodies laid on the pavement under sheets, after the blast hit the number 30 bus at 0947 BST in Tavistock Place, near Russell Square.
The explosion was the fourth to hit the transport network in under an hour.
Officers are investigating whether the blast on the bus, going to Marble Arch from Hackney, was a suicide attack.
Metropolitan Police Deputy Assistant Commissioner Brian Paddick said: "It could as easily be an explosive device left on the bus as the work of a suicide bomber. We are not able to determine which it was yet."
Witness Belinda Seabrook said she was on the bus in front of the one that exploded when she heard an "incredible bang".
"I turned round and half the double decker bus was in the air," she said.
"It was a massive explosion and there were papers and half a bus flying through the air."
Parking attendant Ade Soji, 35, of Dagenham, Essex, described how his colleague was injured in the blast as he ran for safety.
"I think a piece of metal hit him on the head," he said.
Accountant Tim Priestley, 27, from Billericay, Essex, was at his desk when the bus bomb exploded outside his office opposite Russell Square tube station.
"I felt a bang and a shudder. Five minutes later it was all over the TV," he said.
People were evacuated from the nearby buildings and led into the nearby Tavistock Hotel.
'Avoid central London'
The hotel's receptionist, Sam Elliott, said: "People were running in, some obviously injured.
"One man was being carried by other passengers. We just did not know what had happened."
Bus services have been running again, but are subject to diversions.
"Checks are being undertaken on all buses that remain in service and at all bus garages. We are advising passengers not to go into central London," a Transport for London spokesman said.