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Police shut down the area around Parliament on Tuesday after witnesses said a motorist deliberately crashed into pedestrians and cyclists during the morning commute, heightening tensions in a city that has seen four vehicle-based terror attacks in less than 18 months.

Armed police swooped into the area and cordoned off streets surrounding the heart of Britain's government. Police appealed to the public to stay away, and the Westminster Tube station was closed.

Eyewitness Ewelina Ochab told The Associated Press that the crash appeared deliberate.

"The car drove at speed into the barriers outside the House of Lords. There was a loud bang from the collusion and a bit of smoke," she said. "The driver did not get out. The guards started screaming to people to move away."

The area was the site of a terror attack in March 2017 when Khalid Masood ploughed a car into crowds on Westminster Bridge, killing four people. Masood abandoned his car and then stabbed and killed a police officer before being shot dead in a courtyard outside Parliament.

In Tuesday's incident, the car slammed into barriers that had been extended following the Westminster Bridge attack. Video from Sky News showed a man being detained.

London's Metropolitan Police say that the Counter-Terrorism Command is leading the investigation. Though a number of people were injured in Tuesday's incident, none are in life-threatening condition.

The police force says it is keeping an open mind about the incident.