Scotland Yard held a brief press conference to inform the public on what has happened so far.
Apparently, a suspect was seen driving erratically on the nearby Westminster Bridge, hitting bystanders before driving onto the busy sidewalk in front of the gates of Parliament.
Witnesses then said the assailant left the car and rushed through the gates and began attacking police officers with a knife.
Scotland Yard said, "At this stage I will confirm what we know happened but I will not speculate. We received a number of different reports, which included a person in the river, a car collision with pedestrians and man armed with a knife."
He continued, "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. Our response will be ongoing for some time, and it is important that we gather all possible information and evidence. Public safety is our top priority, and we are reviewing our policing stance across London and throughout the capital this afternoon."
He went on, "The acting commissioner Greg Mackey, is being treated as a significant witness as he was at the scene when the incident started. Whilst he is not injured, it would be inappropriate for him to talk about the incidence."
The Yard was being very tight lipped at this point so we'll keep you posted with new developments as they come in.
UPDATE: Scotland Yard has confirmed that four people have died, including a police officer and twenty have been injured.