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 he had seen Nicky Jacobs, 45, stab Pc Blakelock with a machete between two and four times as the frenzied mob took part in a "free for all".
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 screams of 'help, help, help'."
Mr Brown said Jacobs used a machete with a 12-inch blade during the attack.
"He was attacking the officer and hit him with a couple of blows on his body, stabbing up and down with the machete," he said.
Afterwards Mr Brown saw a gash on the officer's neck and blows on his body, he said.
Mr Brown said it was "common knowledge" ahead of the riots there was going to be trouble. He told the jury that he had heard others say that Jacobs was going round saying he was going to "give an officer a hiding if he got the opportunity, or do some damage".
The witness said he had been paid a £5000 reward by the police after giving a statement during a renewed police investigation into the killing in 1993.
Mr Jacobs denies murder.
The trial continues.