A controlled explosion was carried out outside Buckingham Palace after a man was arrested on suspicion of possession of an offensive weapon, the Metropolitan Police said.
The arrest was made at around 7pm after he approached the gates and threw items, suspected to be shotgun cartridges, into the Palace grounds.
Police cordons remain in place surrounding the area.
The incident comes just four days before King Charles' Coronation, with rehearsals scheduled to take place for the procession later this evening.
Some roads in the area had already been closed in preparation.
No shots were fired during the incident, and police say there is no ongoing risk to the public.
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."
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