WA Police Commission Karl O'Callaghan showed he still has what it takes in the field after he ran down and arrested man who he witnessed allegedly assault a woman on Guildford Road, Ashfield, on Tuesday morning.

Commissioner O'Callaghan said he and a passenger were driving along Guildford Road around 10am when they saw a man chasing after a woman on the footpath.

WA Police Commissioner Karl O'Callaghan - said making an arrest was "nothing special".

"He caught up and I saw him assault her," he said.

The police chief said he stopped the car and got out to intervene to protect the victim and to apprehend the offender.