At around 7.12pm officers were in pursuit of a suspect vehicle in Dartmouth Road NW4.
A man decamped from the vehicle and was located by an officer in an alleyway in the Park Road area.
A struggle ensued, and the officer was seriously assaulted.
The suspect fled the scene prior to police back up arriving.
The man [no further details] was located and arrested on suspicion of attempted murder.
The injuries to the officer are not thought to be life-threatening.
A crime scene is in place and enquiries continue.