A man reportedly presented knife injuries to London pub staff before leaving the area, police said.
Officers and London Ambulance staff were alerted to the scene in the early hours of Saturday morning at around 3am in Station Road, Harrow.
Police have arrested a 23-year-old man on suspicion of criminal damage and grievous bodily harm.
He remains in custody at a north London police station.
Police have been unable to locate the man with the reported injuries.
A Met spokesperson said: “Police were called at 03:07hrs on Saturday, 31 July to reports of a group of people fighting in Station Road, Harrow. Officers attended along with London Ambulance Service.
“It was reported that a man had presented to staff at a nearby pub with stab injuries before leaving the area. Officers carried out a thorough search but found no trace of the victim. Enquiries to locate him are ongoing.”
Anyone with information is asked to call police on 101 or tweet @MetCC quoting CAD 880/31Jul.