British home secretary Charles Clarke has called European interior ministers to a special terror summit in Brussels next Wednesday.
He acted yesterday on the suggestion of the Spanish government as members of Europol, the European police intelligence exchange agency, joined British investigators in London last night to help collate information which might enable police to identify those responsible for Thursday's four bomb attacks in which more than 50 people died.
Emergency staff were still retrieving bodies trapped underground more than 36 hours after the blasts.
Police have made no arrests but officials have gradually released more details about the bombings, saying each of the bombs was believed to have contained up to 10 lbs (4.5 kg) of explosives and could have been carried around in backpacks.
The government says the attacks bore the hallmarks of Osama bin Laden and Abu Musab al-Zarqawi's Islamic militant al-Qaeda network, which was behind the September 11, 2001, attacks on New York and Washington.
"We know that there has been direction given by bin Laden to Zarqawi to look to the West and to carry out attacks in Europe and other areas," a senior US counterterrorism official said.
"There isn't a single country in Europe that can consider itself immune to this type of risk," European Union Counter-Terrorism Coordinator Gijs de Vries said in Brussels.
Police said they did not suspect suicide bombers and believed more than one person carried out the attacks.
The investigators were examining a claim of responsibility from the previously unknown "Secret Group of al Qaeda's Jihad in Europe", which said the blasts were punishment for Britain's involvement in Iraq.
World leaders have vowed to defy threats against countries which, like Britain, have troops in Iraq.
Italy said on Friday its troop withdrawal from Iraq would start as planned in September -- and no sooner. Japan said on Thursday it had no plans to withdraw its troops.
British leaders have vowed defiance, and the stoicism of Londoners has been widely praised since the attacks.
The city slowly returned to work yesterday and theatres reopened, but restaurant and bar owners said business was unusually quiet for a Friday evening.
Distraught relatives were still searching for missing loved-ones around hospitals, handing out leaflets asking for information and issuing appeals on television and radio.
"The not knowing is so painful. We just want to know what happened," said Yvonne Nash, hoping to find her missing partner Jamie Gordon.
There was relief for some. Martine Wright, a 32-year-old office worker, was found in intensive care in hospital on Friday after anxious family and friends put out an appeal to find her.
Many of Britain's about 1.6 million Muslims mourned the dead and condemned the bombers at Friday prayers but expressed fears of a backlash.
"Some people will try to instigate anger against Muslims and try to blame us for what happened," said Mohamed Sawalha, who gave the Friday prayers' sermon at London's Finsbury Park mosque.
London police chief Ian Blair said the authorities were in touch with Muslim leaders and those of other faiths to protect symbolic buildings. The government and religious leaders called for a calm response to the bombings after holding talks.