Thousands of people evacuated from Birmingham's city center faced "a real and very credible threat," police Chief Constable Paul Scott (search) said Sunday, while declining to describe the nature of the threat.

Some 20,000 people were evacuated Saturday night because intelligence information suggested a security threat, Scott said.

It was unclear whether the threat had any ties to Thursday's deadly terrorist attacks in London. Stuart Hyde (search), assistant chief constable of the West Midlands police, said Saturday night that there was no evidence linking the two.

The evacuation of the bustling Broad Street entertainment district and the city's Chinese quarter followed an intelligence warning of a "substantial threat," Hyde said.

A controlled explosion — designed to destroy a possible bomb — was carried out on a bus following a call from a member of the public, but officers concluded there was no explosive device on the bus.

Police initially restricted road traffic into the city center, but after receiving further intelligence they ordered an evacuation.

Birmingham, 110 miles northwest of London, was the target of one of the worst Irish Republican Army (search) bombings of the 1970s. Twenty-one people died when the IRA bombed two pubs on Nov. 21, 1974.