Residents in Birmingham are seen sitting on the pavement.

British Police said on Saturday they were evacuating thousands of people from the central area of Birmingham, Britain's second largest city, after receiving intelligence of a threat.

The security alert came two days after London was bombed by suspected Islamist militants that killed over 50 people.

A police spokeswoman said she could not confirm media reports that police had carried out a series of controlled explosions on suspect packages.

A Sky News reporter said she had seen two bomb disposal vans traveling past at high speed, and a police helicopter was hovering overhead.

A police spokesman said: "West Midlands police ... are now closing down the Broad Street entertainment zone and asking people to leave Birmingham town center and go home."

"Our inquiries are ongoing. We're not going into the nature of the threat. ... Clearly we're taking it quite seriously."

He estimated up to 30,000 people could be in the area being cleared, which is packed with nightclubs, bars and restaurants.

No arrests had been made, the spokesman said. "What we're talking about is warning the public and making sure people are safe."

However, clubs, bars and other businesses were allowed to remain open.

"We understand the impact this will have on people and will keep the situation under review and advise the public appropriately," a police statement said.

London has seen repeated security alerts since Thursday's attacks on underground trains and a bus, and police have warned the bombers may still be at large and could strike again.

Birmingham, 110 miles (175 kilometers) northwest of London, was the target of one of the worst Irish Republican Army bombings of the 1970s. Twenty-one people died when the IRA bombed two pubs on Nov. 21, 1974.