A man was arrested outside Buckingham Palace after throwing suspected shotgun cartridges into Palace grounds, police have said.
A cordon was erected and a controlled explosion carried out following the incident, which unfolded at around 19:00 BST on Tuesday.
The man was arrested on suspicion of possession of an offensive weapon. There are no reports of any injuries.
The incident is not currently being treated as terror-related.
Despite the incident, overnight rehearsals for the Coronation on Saturday went ahead as planned.