London police say a man arrested near Buckingham Palace on Friday night was in possession of a four-foot sword.
They are describing the incident as a terrorism, and say three officers sustained minor injuries in the operation.
Just after 8.30pm a car deliberately drove at a police van and stopped in front of a restricted area near Buckingham Palace.
The officers, who were unarmed, got out of the van and approached the blue Toyota Prius car.
File photo of Buckingham Palace
As they approached the driver, he reached for a four-foot sword which was in the front passenger foot well.
The man, who repeatedly shouted Allahu Akbar, was incapacitated with CS spray.
Two of the officers were taken by ambulance to hospital with minor cuts and discharged a short time later.
The 26-year-old man from the Luton area was arrested at the scene on suspicion of grievous bodily harm and assault on police.
Metropolitan Police say its Counter Terrorism Command unit are now leading the investigation.
The man remains in custody at a London police station.
