A controlled explosion was held and a man was arrested outside Buckingham Palace after throwing suspected shotgun cartridges into palace grounds, police have said.
A cordon was put in place after the man was detained at around 19:00 BST on Tuesday after approaching the gates of the palace, Scotland Yard said.
He was arrested on suspicion of possession of an offensive weapon.
There are no reports of shots fired or injuries to officers or the public.
Police said they are not treating the matter as terror-related.
It is understood it is being treated as an isolated mental health incident.
Police later said the man was searched and a knife was found, but he was not carrying a gun.
The arrest comes just four days before the King’s Coronation celebrations – which will be attended by world leaders and other royals from around the world.
The man was also found to be in possession of a suspicious bag, the Metropolitan Police said.
A controlled explosion was carried out as a precaution after an assessment from specialists.
The King and the Queen Consort were not at Buckingham Palace at the time of the arrest.
Chief Superintendent Joseph McDonald said: “Officers worked immediately to detain the man and he has been taken into police custody.
“There have been no reports of any shots fired, or any injuries to officers or members of the public.
“Officers remain at the scene and further enquiries are ongoing.”
The BBC’s royal producer Sarah Parrish was in the broadcast compound outside Buckingham Palace when she was told to leave and wait outside.
She told the BBC News Channel that those who were evacuated “heard the controlled explosion and then we were allowed back in again.”
The suspected shotgun cartridges have been recovered and will be examined by specialists.
A ring of steel is expected in the capital for the Coronation at Westminster Abbey on Saturday.
Policing minister Chris Philp has described it as a “huge policing operation”.