This happened in Charleroi, Belgium. The attacker was armed with a knuckleduster.

The story. They were skating when this chav enters. He asks the skater if he could try out his skateboard and the skater agrees.
But this chav grabs the bord a slams it against a wall. The skater's reaction: "ehhh c'est pas cool", makes him go crazy and that's when the vid starts.

The funny looking elderly man is saying: "Don't release him, he's dangerous!"
The cop is actually only trying to calm him down.