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'."
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Patrols including local volunteers have been set up after the grisly finds in Thamesmead in the last month.
Thieves targeted a property in Richmond on Monday afternoon.
The station, which saw 44m people commuters use it in 2016/2017, will be transformed into a dining room complete with Christmas decorations.