Scotland Yard have confirmed there is an incident involving armed police officers at University College Hospital in Bloomsbury close to Warren Street station.
A man was also arrested by armed police at the gates of Downing Street.
An internal memo to staff at the hospital has warned them to be on the lookout for a black male, possibly of Asian origin, about 6ft 2ins tall, wearing a blue top with wires protruding from the rear of the top.
Police officers in chemical protection suits have been seen appearing to enter Warren Street, where British Transport Police say one person has been injured.
A initial chemical search at Oval station has proved negative.
Sir Ian told reporters at Scotland Yard the incidents were "serious".