Constable Tisshaw, 39, who lives in Kent and is based at Croydon Traffic, was on motorcycle traffic patrol duty in Brixton when the riot erupted on Wednesday night.
"At first they were doing what demonstrators do, which is move about a little bit. It was no different from any other demonstration," he said from his bed in St Thomas's Hospital, London.
"We started hearing on the radio that they had set fire to the 7-Eleven store and that looters had gone by ... the crowd moved down Brixton Road. We were just leaving and 20 or 30 of them just appeared from I don't know where.
"They attacked from behind and from beside. They dragged me off the bike and I ended up in a ball on the floor.
"I got kicked - fortunately I had a crash helmet on because I felt two or three kicks to the head and I got kicks on the back. Someone took my baton off me and used that on me, I am led to believe.
"In that situation seconds feel like hours, to be honest. My colleagues came to my help and one got a kick in the groin. I saw people coming back to my help - they were half a dozen members of the public - to help me out, black and white, although I don't want to make a point of it," he said.
He remembered being dragged out of the melee but did not recall much more until he was being tended by a police medic.
He said he had been told of reports that a motorist drove at the crowd, forcing it to disperse and almost single-handedly preventing a potential murder. "If I meet him I would be very thankful for what he did."
PC Tisshaw also praised the Territorial Support Group for pulling police vans up on each side. He said: "While one of their medics was looking after me, there were four of them whose job was just to fend off the bricks. How do you say thank you for that?"
Asked whether he feared for his life, PC Tisshaw, who suffered a broken shoulder, fractured ribs and bruising in the attack, said: "They did serious harm to me. But whether they would have killed me, I don't know. You are in the lap of the gods at that stage of a beating."
PC Tisshaw, who has been in the Metropolitan Police for 20 years, and was in Brixton on the night of the last riots 10 years ago, said he expected to return to duty when he had recovered.
His wife of 19 years, Lynda, visited him after the attack and was expected to be at his bedside again today.
Asked what he felt about his attackers, he said: "I don't know at this stage if I feel outrageously angry towards them. They are just faceless.
"It is just something that has happened, and they will or they will not get brought to justice."