The evacuation came amid heightened tensions in Britain since the Thursday morning rush hour bombings on London's transport system in which at least 50 people were killed and some 700 injured.

Police early Sunday lifted a cordon around the city center, including the night life hub, after bomb disposal squad officers carried out four controlled explosions on a bus before ruling a second suspect package harmless.

Paul Scott-Lee, chief constable for the West Midlands Police, told reporters Sunday that police had acted on a "credible threat" received by intelligence services that was "was specific about the time and also the locations." But Scott-Lee would not elaborate on the nature of the alarm.

He put the threat in the context of the broader global climate on terrorism, saying it was unrelated to the suspicious packages which were found and turned out to be harmless. "It was not a false threat. It was a serious threat. The intelligence indicated that the people of Birmingham were in danger last night. And we responded to make sure that the people were taken out of the danger area," he said.

Police had lifted the cordon on incoming traffic to the inner ring road of Britain's second-city but confirmed searches were ongoing. They said the threat was probably not connected to Thursday's bomb attacks in the British capital and that the decision to close a large part Birmingham and evacuate an estimated 20,000 people had not been taken lightly.

Police searched bars, restaurants and clubs in an area known as the Golden Mile, frisking patrons as they came and went.

London death toll

Britain's interior minister, Charles Clarke, said on Sunday that he expected the death toll from the London bomb attacks to rise and that he was optimistic the perpetrators eventually would be caught. However, he said he feared more attacks if they remained at large.

"We think it's likely to be more than 50" people killed, Home Secretary Clarke told BBC television. Several dead have yet to be recovered as crews work to clear the wreckage of some underground train carriages. More than 20 people are still missing.

Police said on Saturday that they had made no arrests and had yet to focus on any particular suspect. "I am confident that the perpetrators will be brought to justice in the end," Clarke said. "I am very optimistic indeed. I think the track record of our security services in bringing people to justice is good."

But he said he feared the possibility of further attacks if the bombers remained at large. "That is why the number one priority has to be the catching of the perpetrators," he said.

Home grown?

The former head of Scotland Yard warned on Sunday that the London bombers were probably British and that there were many people in the country willing to take part in such atrocities.

"The terrorists at the center of the London bombing this week will almost certainly be British born and bred, brought up here and totally aware of British life and values," Sir John Stevens wrote in a column for the News of the World tabloid.

Stevens, who quit his job as Britain's most senior police official early this year, dismissed speculation that the terrorists had come from abroad. "That's just dangerous wishful thinking, a damaging illusion," he wrote. "I'm afraid there's a sufficient number of people in this country willing to be Islamic terrorists that they don't have to be drafted in from abroad."