Police sealed off a busy central London street during rush hour amid reports of an armed man causing a disturbance, but said the incident was not terrorism-related.
TV reports say four people have been held hostages by a man who is reportedly carrying a bomb on him.
At least a dozen emergency vehicles with lights flashing could be seen blocking off Tottenham Court Road in London, AFP reporters said.
"We were called at 1159 (around 1100 GMT) to an office in Tottenham Court Road. There were reports of a man causing a disturbance. It is alleged that he is throwing items from a fifth floor window," a Scotland Yard spokeswoman said.
"We have a negotiator on the way to the scene and a 100-metre cordon in place. We do not believe it is terrorism-related."
The spokeswoman would not comment on reports that the man had gas canisters strapped to his body and was threatening to blow himself up.
Staff at the Huffington Post website, whose offices are nearby, said there was a 'hostage situation' and that armed police were at the scene.