Eyewitness 'John Brown', a former member of the Park Lane Boys gang, who accepted a caution for assaulting his partner in 2010, said defendant Nicky Jacobs used a machete with a 12-inch blade during the attack on Pc Keith Blakelock.
He told the jury that he himself had kicked the officer some 10 times after he went to the ground.
Asked why, he said: "Looking back on it I don't know...It was the excitement of the situation, I just rushed forward and kicked him about ten times."
The witness said he had been paid a £5,000 reward by the police after giving a statement during a renewed police investigation into the killing in 1993.
Asked by Richard Whittam QC, for the prosecution, what the officer was doing during the attack, Mr Brown said: "What I saw, he was trying to curl up into a ball, I suppose to protect himself somehow.
"Did he say anything?" Mr Whittam asked.
"I just heard heard screams of 'help, help, help'."
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".
The sergeant in charge of PC Keith Blakelock's unit has described the "terrifying" moment they came under attack by a mob with iron bars and a machete. David Pengelly led his team of 10 officers to protect firefighters tackling a blaze at a supermarket.
It was a scene of considerable disorder. There were lines of officers with shields. There were missiles being thrown. It was a scene of considerable rioting.
I saw a group of youths. My eye was drawn to youths who were running underneath us. A group appeared at the far side of the level we were on from an entrance of some sort.
– David Pengelly
There was something perhaps of a dramatic pause. I do not know the reason. Then another group appeared slightly to the right from an unknown point to me.
There was no time to reflect as the second group immediately starting throwing items at us - bottles, bricks, I believe mainly.
It's alleged the suspect in the PC Blakelock murder wrote a rap about the killing while serving a sentence for affray. Nicholas Jacobs is alleged to have written:
"Me have de chopper we have intention to kill an police officer PC Blakelock..."
The rap went on "he beg for mercy but it didn't matter.. him get capture. Him have to face consequences."
The trial into the murder of PC Blakelock during the Broadwater Farm riots in 1985 is underway.
The prosecution claims the defendant was armed with a knife and was part of a group of 15-20 people in the inner circle of those attacking PC Blakelock. It's also outlined the two previous investigations into his murder.
The trial of the man charged with the murder of PC Keith Blakelock during the Broadwater Farm riots in 1985 is about to begin at Old Bailey. The man charged, Nicholas Conrad Jacobs, denies a single count of murder.
The jury is made up of 7 men and 5 women. It is alleged that the defendant was armed with a knife and was part of the group that attacked PC Blakelock.
PC Blakelock's family has arrived at the Old Bailey, as Nicholas Jacobs stands trial today for his murder during the Broadwater Farm riots in north London in 1985.
A very visible, vocal group outside the Old Bailey on behalf of Nicholas Jacobs. He denies murdering PC Keith Blakelock in the Broadwater Farm riots of 1985. The trial starts this morning.