A witness, known as Rhodes Levin to protect his identity, told the Old Bailey, London, he had seen Nicky Jacobs, 45, attacking the officer on the estate in Tottenham on October 6, 1985. Jacobs denies murder.
Levin, who himself admits kicking the police officer "a few times", said he saw Jacobs kicking and punching Constable Blakelock during the onslaught.
He also claimed he had seen the defendant carrying a lock knife at the scene.
And he said that immediately afterwards Jacobs had told him he had "got a couple of dukes in" on the officer.
Levin told the court that at that point he saw a police helmet but could not remember who was holding it. "They were passing it around, showing it," he said. "Like I say, it was like a trophy."
The witness said that days after the attack he had met Jacobs, who again boasted of getting "a couple of stabs" in on the officer.
Levin was sentenced for affray and handling stolen goods in relation to the riots.
He was paid about £5000 and given immunity from prosecution after providing information in the 1990s to a renewed investigation into the killing. Since 2008 he has received money for a deposit on accommodation and expenses., the court heard.
The trial continues.