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'."
More top news
A 24-year-old man has been arrested on suspicion of murder after the body washed up on the Thames shoreline.
Seven people have now been arrested after a man died from serious head injuries
The 38-year-old officer was on his way home from a night shift when he collapsed. His death is not being treated as suspicious.