Officers quickly detained a man at around 7pm on Tuesday, 2 May after he approached the gates of Buckingham Palace SW1 and threw a number of items – suspected to be shotgun cartridges – into the Palace grounds.
These have been recovered and will be taken for specialist examination.
There have been no reports of any shots fired, or any injuries.
The man has been arrested on suspicion of possession of an offensive weapon and will be taken into custody.
Cordons are in place after the man was also found to be in possession of a suspicious bag.
Specialists are in attendance and will assess the item.
Police have carried out a controlled explosion on the item and the PM has called a Cobra meeting for 9pm this evening following the arrest of the man