Police were called by London Ambulance Service [LAS] shortly after 10.30pm on Sunday, 10 January, following reports of a fight on Field End Road, Pinner.
Officers attended, along with LAS, and found an unresponsive man. Despite the best efforts of emergency services, the man, believed to be aged in his 40s, was pronounced dead at the scene.
His next of kin have been informed but formal identification is yet to take place.
The death is being treated as suspicious. A post-mortem examination is scheduled to commence on the afternoon of 11 January.
A crime scene remains in place as enquiries continue to establish the circumstances. The Met’s investigation is led by homicide detectives from Specialist Crime.
A 21-year-old man was arrested in connection with the incident and is currently in police custody.