British police have arrested three terrorist suspects at Heathrow airport, but said they had no reason so far to link them to last week's bombings in London.

"Three people have been arrested under the Prevention of Terrorism Act at Heathrow airport," Scotland Yard Deputy Assistant Commissioner Brian Paddick told a news conference on Sunday.

The arrests were made earlier in the day; but he did not say whether the suspects were entering or leaving the country.

Paddick said it would be "pure speculation" to link them to last Thursday's bombings on three underground trains and a bus, from which police have recovered 49 bodies and are still retrieving more from a tunnel below King's Cross station.

He later told Sky Television: "My understanding is these are reasonably routine arrests under the Prevention of Terrorism Act ... there is no connection that we know of at this stage."

Anti-terrorism laws

Britain has detained more than 700 people under anti-terrorism laws since the 11 September 2001 attacks on the United States.

Of these, around 120 have been charged with terrorism offences and another 135 under other legislation.

Police have so far not reported any arrests directly linked to the attack on the capital, which the government says bears the hallmarks of the al-Qaida network. 

They have been at pains to stress that the inquiry will require slow and meticulous work, including the gathering of forensic evidence from the four bomb sites, three of which are in subway tunnels.

Paddick said police had received important information from members of the public, who had placed 1700 calls to a special investigation hotline.

Birmingham alert

"A considerable number of these calls are proving to be very, very valuable to us," he said.

Police say the three subway bombs went off almost simultaneously, making it more likely they were detonated by timers, rather than bombers. That means the bombers may still be at large and could strike again, they said.

Meanwhile, a huge overnight security alert in Britain's second city of Birmingham kept Britons on edge on Sunday.

Police evacuated 20,000 people from the city centre on Saturday night and carried out four controlled explosions on a bus. They found no bombs but said the drastic measures were fully justified.

"The threat that we responded to yesterday was very specific," West Midlands police chief constable Paul Scott-Lee told a news conference on Sunday.

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"It was specific about the time and also the locations ... The people of Birmingham were in danger last night."

Meanwhile, in London, anxious relatives continued to scour hospitals in search of loved ones missing since Thursday's blasts - the worst peacetime attacks on the British capital.

Walls, bus stops and telephone boxes close to King's Cross station, scene of the worst blast, were covered with photographs of missing people and appeals for information about them.

Well-wishers have left hundreds of bouquets of flowers outside the station, many accompanied by messages testifying to London's multi-ethnic and multi-national mix.

"Christian, Muslim, Jewish, Hindu, Sikh, Buddhist. We are all Londoners," read one message scrawled on a Union Jack flag.

"Our prayers are with you. Keep the faith. From all South Africans," read another written on that nation's flag.

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