A Met Police officer "unintentionally" fired his gun while attending an incident involving the force's kidnap unit in west London.
It happened when kidnap and firearms officers were involved in an operation in Uxbridge earlier on Saturday.
The Met said there was an "unintentional discharge of a firearm" and no one was injured as a result.
A man was found with multiple stab injuries at the scene and taken to hospital in a "serious condition".
Two men have been arrested in connection with the operation, which the police refused to comment on.
The Directorate of Professional Standards has been informed of the firearms incident.
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