Reports also suggest that their might be 4 hostages held at the scene.

Scotland Yard said in a statement that police were called to Tottenham Court Road, a busy central London avenue, just before noon and had sent a hostage negotiator to the site after reports had circulated that the man was holding four hostages in the building. Office supplies, including computer monitors, were being thrown from the building's fifth floor window.

"It's unclear if other people (are) in the building," police said in a statement.

The Huffington Post, whose offices are just off Tottenham Court Road, said that its building was among those evacuated and quoted an eyewitness as saying that a man had walked into her office "strapped up in gasoline cylinders."

"Basically he threatened to blow up the office," Abby Baafi, 27, told the news website.

"He said he doesn't care about his life, he doesn't care about anything, he's going to blow up everybody."

Baafi, whom The Huffington Post identified as an employee of training company Advantage, said she recognized the man as a former customer.

"He's not quite stable, not mentally stable," she said.

A telephone message left with Advantage wasn't immediately returned.

Reporters at the scene state that they saw heavily armed policemen in balaclavas taking up positions within the cordoned-off area around Tottenham Court Road. (AP)