A man has been arrested on suspicion of attempted murder after two Ministry of Defence police officers were confronted with a knife.
The incident happened at about 08: 50 BST at Horse Guards Parade in Westminster, London.
A Taser was used and a 29-year-old man was restrained by officers, the Met Police said.
There were no reports of any injuries and the incident was not being treated as terror-related, the force said.
The man was also arrested on suspicion of possession of an offensive weapon and remains in custody.
Whitehall was closed for several hours due to the incident.
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