While one officer pinned the man down with a knee on his neck, his colleague fired a Taser at the inside of the man's left thigh at point-blank range.
The man yelled in agony before the other cop landed four punches in his face.
As around 30 people booed and jeered at the violent scenes, one of the officers shouted: "He's resisting arrest."
But marketing student Chris Parsons - who filmed the incident in Swansea, South Wales, on his mobile phone - said: "It was shocking and really over the top. The cops were just talking to him and then all of a sudden for no reason it just kicked off.
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