Several people were injured, but their conditions were not initially known.
“At this early stage none of the injuries appear to be life-threatening,” Metropolitan Police said.
Authorities say the man driving the vehicle was immediately arrested. He wasn’t initially named but officials said he’s in his 20s.
Police added “nobody else was in the vehicle, which remains at the scene and is being searched,” and said no weapons have been recovered.
Investigators are treating Tuesday’s incident as terrorism and the counter-terror division is leading the inquiry.
The scene was cordoned off as officers began the investigation and the Westminster Tube station was closed.
“It’s inevitable it’s going to be investigated as an attack in view of the location of the incident. And certainly the initial assessment of the incident doesn’t seem like a normal traffic accident,” former Met Police chief inspector Peter Kirkham told Sky News.
“The eyewitness statements all seem to point towards it being a deliberate attack.”
“Our thoughts are with those who have been injured in the incident outside the Houses of Parliament this morning, and our thanks go out to the quick and effective response of our police and medical service,” British lawmaker Vince Cable tweeted. “Thank goodness the barriers are in place and worked.”
“My thoughts are with those hurt and injured outside Parliament this morning in what is being treated as a terrorist incident,” Labor Party leader Jeremy Corbyn tweeted.
“Our thanks go to our emergency services who responded immediately. Their bravery keeps us safe day in, day out.”