A man found with a large blade outside Buckingham Palace is being questioned by counter-terror police.
Two officers were injured after tackling the 26-year-old near the Queen's official London residence on Friday night.
The suspect was arrested at the scene on suspicion of grievous bodily harm and assault on police and has since been further arrested under the Terrorism Act.
The two policemen received minor injuries to their arms as they arrested the man, after spotting a weapon in his car.
They have since been discharged from hospital, the force said.
The suspect, who stopped his car in a restricted area near a police vehicle on the Mall roundabout shortly after 8.30pm, was also taken to hospital to be treated for minor injuries.
He is now being questioned by detectives from the Metropolitan Police's Counter Terrorism Command at a police station in the capital.
Police had previously described the weapon as a knife but have now said it is a "large bladed weapon".
One woman said her partner had seen the blade and initially thought it was a sword.
The passer-by, who asked not to be named, said: "My partner saw a sword, which I didn't see, as well as a policeman with blood on him, looking like his hand or chest was injured.
"The police officer had it in his hand, walking away with it."
It is understood that no members of the Royal Family were in Buckingham Palace at the time.
Detective Superintendent Guy Collings paid tribute to the "quick and brave" actions of the officers.
Witness 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 eyewitness had said that he had driven towards the police car.
"The whole encounter lasted around one minute."
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 in the wake of the incident.
People filed out of the affected area, saying they had been directed to leave.
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."
Police tape closed the entrances at both Buckingham Gate and at Wellington Arch on Friday night, while Birdcage Walk and The Mall - the roads running around St James's Park - remained taped off.
Police said there is still a cordon at the scene.
Scotland Yard has appealed to anyone who saw the incident or who has video footage or pictures to contact them.