A man armed with a knife has been arrested outside Buckingham Palace, leaving two police officers injured, Scotland Yard said.
The suspect is being held on suspicion of grievous bodily harm and assault on police after being stopped in possession of the weapon at around 8.35pm on The Mall.
One witness described seeing the man wrestled from a car by police having apparently driven his vehicle towards them.
Kiana Williamson told the Press Association: "We turned up and there was one police van and one car, there was also a civilian's car that had veered towards the police car.
"They were trying to get the man out of the car, shouting, more police were arriving on to the scene and the man was fighting back.
"I saw one injured policeman with an injury to his arm although it didn't look severe.
"He was being tended to by another officer.
"The man had been restrained and looked almost unconscious by the side of the road.
"I didn't see the car driving but the car had been left at the side of the road and an eye witness had said that he had driven towards the police car."
The whole encounter lasted around one minute, she added.
The Metropolitan Police said it was too early to know whether the incident was terror related.
It is understood that no members of the royal family were in Buckingham Palace at the time.
Two male officers suffered "minor injuries" to their arms during the arrest, according to the force.
The London Ambulance Service attended to treat the injured men but they were not taken to hospital.
No other people were injured.
Armed police patrolled the entrances to the palace where several police cars and at least two ambulances could be seen.
People filed out of the affected area, saying they had been directed to leave.
An American couple outside the palace, who did not give their names, said they saw a car being searched by officers.
Witness Nicole Kyle was walking home when she saw armed police swoop on the scene and quickly put up a cordon.
The 25-year-old consultant said: "I was walking toward The Mall and St James's Park when I saw police rush down toward the palace in great numbers.
"As we got further down The Mall we saw armed police outside a police van, at which point we were able to walk closer to the palace still, where we were eventually stopped by a cordon.
"All we were told was that there's been an incident.
"A few minutes later police came by to have us clear the area and move further away from the palace.
"We then saw that police had expanded the cordon. It looks like the cordon is continuing to expand."