PC Blakelock curled up in a ball and screamed for help as a mob hacked him to death during the Broadwater Farm riots, an eyewitness has told the Old Bailey.
Using the pseudonym John Brown and with his voice distorted by a modulator to protect his identity, the witness described how he saw the killing on the estate in Tottenham, north London, on October 6 1985.
He explained that he had seen Nicky Jacobs, 45, stabbing PC Blakelock with a machete between two and four times as the frenzied mob took part in a "free for all".
More top news
The memorial will focus on remembering the 71 victims of the tower block fire.
A woman died as she crossed the road when she was struck by four separate motorists who police believe did not stop.
Patrols including local volunteers have been set up after the grisly finds in Thamesmead in the last month.