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HORROR: Armed man 'threatens to blow himself up in office'

Police snipers and armed officers put several blocks of central London's shopping district into lockdown after a man allegedly armed with gas canisters entered an office building and threatened to blow himself up.

Police said a 49-year-old man was arrested hours after a stand-off with police near London's Tottenham Court Road.

A shirtless man in green khaki pants was seen being led out with his hands behind his back by two unarmed officers.

Police and security personnel said the incident was not terrorism-related and presented no risk to the Olympics.

One witness identified the suspect as a former customer and said he had come in "strapped up in cylinders".

"Basically he threatened to blow up the office," Abby Baafi (27) said.

Police declined to say whether he had been armed.

The Huffington Post's offices were one of several buildings in the area evacuated by police.

Several blocks were cordoned off and underground stations were closed as office equipment was hurled from a fifth-floor window.

"He threw 10 computers out of the window," 26-year-old construction worker Martynas Vristiuk said, adding that one of the computers landed on a police car.

The three-hour-long siege turned the normally bustling Tottenham Court Road into a ghost town.

The motive behind the standoff wasn't immediately clear. Police Commander Mak Chishty, who gave a brief statement following the arrest, identified the suspect only as "a local man" and said the incident was over a "quite local" matter.

"This was an isolated incident and has no bearing on security preparations ahead of the summer games," said a British security official on condition of anonymity because of the sensitivity of his work.