"He wasn't going to give up the bag willingly. I tried to ram the bike with my car but I wasn't able to.
"They slashed him in the face with the knife and made off down the King's Road. I tried to follow them but I lost them as they were jumping red lights."
The witness called the police moments after one of the thugs pulled out a knife and started attacking the helpless victim.
He also spoke of how Daniel Radcliffe was quick to help.
He told the paper: "He was a really nice bloke, a lot of stars wouldn't have stopped to help. The victim was very, very shaken up."
A spokesman for the Met Police said: "Police were called at approximately 6.10pm on Friday, July 14 to reports of a robbery in Hortensia Road.
"The victim - a man aged in his 50s - was walking in the road when two people riding a moped snatched his bag before riding off. The victim suffered a cut to the face during the incident.
"Officers from Kensington and Chelsea investigate. There have been no arrests made and inquiries continue."
A spokeswoman for Daniel Radcliffe confirmed to the Evening Standard that he was at the scene.
This article originally appeared on the Daily Telegraph.