David Videcette told the Standard he noticed the men behaving suspiciously and began following them, intervening in the attack by shunting the moped with his vehicle after seeing them pull out a knife.
The man was left with facial injuries and Radcliffe was on the scene consoling the victim. He said, "It was a bit of the surreal moment".
"He was a really nice bloke, a lot of stars wouldn't have stopped to help", Videcette added.
Former police detective David Videcette was on the scene, providing his account of the suspicious moped drivers and the aftermath, saying, "The victim was very, very shaken up".
A rep for Daniel confirmed the actor was present but refused to provide further comment.
It is the latest in a spate of incidents involving criminals riding mopeds following five linked acid attacks in east London last week.
He told the Evening Standard: 'They grappled with him [the victim] on the pavement. He wasn't going to give up the bag willingly.
A Met Police spokesperson told the newspaper: "Police were called at approximately 6.10pm on Friday, July 14 to reports of a robbery in Hortensia Road".
Unfortunately, the robbers did get away with snatching the man's bag and not only that, the victim, in his 50s, was slashed in the face. Officers from Kensington and Chelsea investigated.
"No arrests were made and enquiries continue".
Quizzed on whether he would say that "Hollywood has been - or is - racist", Radcliffe replied: "I mean, yes - I think it's pretty undeniable".