Nicky Jacobs, 45, is accused of stabbing Pc Keith Blakelock, 40, as the officer tried to protect firefighters tackling a blaze at the height of the unrest in on the Broadwater Farm estate in north London in 1985.
A jury at the Old Bailey was told this was the second trial over Pc Blakelock's murder, the first set of convictions being quashed on appeal.
A fresh investigation into the attack led to the decision to give immunity to some people who admitted kicking and punching the officer, with police focusing on those who attacked with weapons, the court was told.
Rather than facing murder charges, some rioters will be giving evidence under pseudonyms because only those in the "inner circle" of the crowd around the officer were close enough to see.
Opening the prosecution at the Old Bailey, Richard Whittam QC warned jurors to treat the witnesses' evidence with care.
He also disclosed that some witnesses had been paid rewards for their co-operation. He told the jury: "Your task is likely to be centred on deciding whether or not you can be sure that these witnesses are telling the truth when they tell you that Nicky Jacobs took part in the attack with a bladed article, acting with others."
Mr Whittam described how the riots erupted on the night of October 6 1985, the day after a suspect's mother, Cynthia Jarrett, had a heart attack and died during a police search of her Tottenham home.
Pc Blakelock was among a group of uniformed officers sent out without cover to protect firemen.
"Fearful they may become trapped both the police and the firefighters were forced to retreat. Outside the flats, as they ran for safety, Pc Blakelock and Pc Richard Coombes went to ground and were set upon to shouts of 'kill the pig' and the like.
"Pc Coombes was very fortunate to survive. Pc Blakelock did not. The attack on him was without mercy."
Jacobs denies murder. The trial continues.