Members of the emergency services take an injured person away on a stretcher after an incident on Westminster Bridge in London on March 22.
British police said there had been an unspecified number of casualties including police officers in a "terrorist incident" close to Britain's Houses of Parliament on Wednesday.
"We know there are a number of casualties including police officers but at this stage we cannot confirm numbers or the nature of these injuries," Commander BJ Harrington told reporters.
"We received a number of different reports which included a person in the river, a car in collision with pedestrians and a man armed with a knife."
He said the acting head of London police, Craig Mackey, was being treated as a significant witness as he was at scene when the incident took place. He was not one of those injured.