A man has been arrested after he allegedly attacked police with a knife outside Buckingham Palace.
Two police officers suffered minor injuries to their arms as they overpowered and detained the armed man.
Armed police officers have set up a large cordon as they patrol the area around the Queen's residence in central London, with witnesses saying the area was in "lockdown".
A witness claimed on Twitter that the attacker was armed with a sword, while another claimed he had a machete.
Witnesses described seeing the man wrestled from a car by police after apparently driving his vehicle towards them and hitting a bollard.
The Queen was not at Buckingham Palace when the incident unfolded in front of shocked tourists at about 8.35pm. She is currently away at Balmoral Castle, her holiday home in Scotland.
The Metropolitan Police said it was too early to know whether the incident was terror related.
Witness Kiana Williamson said: "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."
An onlooker at the scene said: "He hit a bollard and got out with a machete and no members of the public have been injured."
The whole encounter lasted around one minute, she added.
Witnesses said a car was being searched by police.
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."
A Met Police spokesman said: "A man has been arrested on suspicion of grievous bodily harm and assault on police.
"The man was stopped this evening at approximately 8.35pm by officers at the Mall outside Buckingham Palace in possession of knife.
"During the course of detaining him, two male police officers suffered minor injuries to their arm.
"The London Ambulance Service were called and they were treated at the scene. They have not been taken to hospital."
The spokesman added: "No others persons have been reported injured.
"Officers remain on scene and enquiries are ongoing."
There was a heavy police presence around Buckingham Palace, one of the UK's most popular tourist attractions, in the aftermath.
The man was arrested at the top of The Mall, not far from the royal residence's gates, said witnesses.
Several pairs of armed officers were stationed at checkpoints around the palace, as pedestrians and cars were turned away.
Witness Amir Jan Malik wrote on Twitter: "Being directed away from Buckingham Palace now as Police corden [sic] it off. No entry allowed in towards palace."
He added: "Stuck outside Buckingham palace with many armed police officers. Heavy rifles on display traffic at a halt."
Another witness, Leonardi Paoli, wrote: "Buckingham palace in lockdown. Man with a sword attacker an officer."
He said he overheard police discussing a "sword attack" following the incident.
A Buckingham Palace spokeswoman said the palace does not comment on security issues.
The incident occurred just hours after a man was fatally shot in Brussels after attacking two soldiers with a knife.
The soldiers were lightly wounded in the attack, which comes as European cities are on high alert due to the ongoing terror threat.
In response to the London incident, Lib Dem leader Sir Vince Cable tweeted: "Thoughts and sympathies with brave officers injured tonight at Buckingham Palace in this unprovoked and cowardly attack."
Man 'armed with sword attacks police' outside Buckingham Palace as two officers injured during arrest - Mirror Online