Counter terrorism police are investigating after a man with a "large blade" was arrested outside Buckingham Palace on Friday (August 25).
It is believed the man had stopped his car near a police vehicle on the Mall roundabout at around 8.35pm.
Scotland Yard said two police officers suffered "minor injuries" while detaining the man, thought to be in his 20s, after he was spotted with the weapon.
Counter terror police are questioning the suspect following the arrest.
He was arrested on suspicion of grievous bodily harm and assault on police and has since been further arrested under the Terrorism Act.
The suspect, who stopped his car in a restricted area near a police vehicle on the Mall roundabout, 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".
Armed police officers set up a large cordon as they patrolled the area around the Queen's residence in central London, with witnesses saying the area was in "lockdown".
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.
Detective Superintendent Guy Collings said: "A man in his mid-20s has been arrested by police after they spotted a weapon inside his car.
"The quick and brave actions of both officers meant that the suspect was detained very quickly.
"No members of the public had any interaction with this individual at the scene. He will now be questioned by detectives in police custody. It is too early in this investigation to speculate any further."
Witness Kiana Williamson said: "We turned up and there was one police van and one car.
"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 whole encounter lasted around one minute."
Witnesses said a car was being searched by police.
London Ambulance Service attended to treat the injured men, who 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."
Police tape closed the entrances at both Buckingham Gate and at Wellington Arch.
Birdcage Walk and The Mall - the roads running around St James's Park - remained taped off.
People were seen walking out of the cordons, but no-one was allowed in.
Several pairs of armed officers were seen stationed at checkpoints around the palace.
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."
Keep up to date with the latest news in west London via the free getwestlondon app.
You can set up your app to see all the latest news and events from your area, plus receive push notifications for breaking news.