Video footage of the incident shows officers surrounding the auto and arresting one man.
It appears to have been a deliberate act on an iconic London building, and as such, was being treated as a terrorist incident, said Metropolitan Police Assistant Commissioner Neil Basu during a news briefing later Tuesday.
"At this early stage none of the injuries appear to be life-threatening".
Surrounding roads including Westminster Bridge, Abingdon Street and Whitehall all closed today, along with Westminster station on the London Underground. "Officers remain at the scene", it said.
A man has been arrested and several people injured after a vehicle crashed into barriers outside the UK Houses of Parliament.
Parliament Square has been closed as a result of the crash. The BBC says it drew a major response by "armed police, ambulances and firefighters".
Images posted to social media showed a suspect surrounded by officers being led away from the vehicle in handcuffs.
Police also said a "number" of pedestrians were injured.
The auto slammed into barriers that had been extended following the Westminster Bridge attack in March 2017 when a British convert to Islam ploughed a vehicle into crowds on Westminster Bridge, killing four people.
Parliament is now on summer recess, and isn't due to reconvene until early September, meaning the Houses are fairly quiet at the moment.
"It didn't look like an accident". In March 2017 Khalid Masood rammed crowds along Westminster Bridge, killing four people.
Last year Khalid Masood murdered five people, including a police officer and injured 50, when he ploughed his vehicle into pedestrians in a terrorist attack.
London Ambulance said it had sent a number of ambulance crews to Westminister and that two people had been taken to hospital for non-serious injuries. "There was a loud bang from the collusion and a bit of smoke", she said.
The driver, who is in his late 20s, has been arrested on suspicion of terror offences and is being held in custody at a police station in South London.
Although no motive was immediately identified in Tuesday's incident, Britain's capital has been subject to a series of terrorist attacks in recent years.