A police officer accused of killing Ian Tomlinson during the G20 protests was "shocked" when he first saw film footage of him pushing the 47-year-old to the ground, a court has heard.
Inspector Timothy Williams told Southwark Crown Court that Pc Simon Harwood, 45, had his head in his hands when he saw the clip being shown on television.
Harwood hit Mr Tomlinson with his baton and shoved him to the ground near the Royal Exchange Buildings in the City of London in April 2009.
Mr Tomlinson walked around 75 yards before he collapsed and later died.
Harwood denies manslaughter on the grounds that he used reasonable force.
Inspector Williams said: "He watched it and he said to me that he thought he was the officer concerned.
"He had his head in his hands, he was facing towards the ground and sort of looking up to me. He was obviously shocked."
At that stage senior officers believed that someone else was involved.
But Harwood, 45, from Carshalton in Surrey, insisted that he was not "chomping", which means joking.
Insp Williams said: "There is an expression called 'chomping' which is used within the TSG (territorial support group) which is joking. By somebody saying I'm not chomping, it means 'no I'm serious, I think this is me'."
In cross examination by Patrick Gibbs QC, for Harwood, the court heard that a message had come from senior City officers to treat lawful protesters "with kid gloves" but to deal with violent law breakers "with an iron fist".
Jurors were told that the approach was: "Treat the lawful protesters with kid gloves but deal with those using violence with an iron fist."
Mr Williams agreed that Harwood had looked "horrified" when he watched the film clip.
In his own notes about that day, Harwood said that before he came across Mr Tomlinson, other protesters were rioting and throwing missiles at him.
He unsuccessfully tried to arrest a man for writing ACAB - short for "all cops are bastards" - on a police carrier, and found around 100 demonstrators chasing him.
Harwood, who had no riot shield, said he was in fear for his safety and began hitting protesters with his baton because they were trying to punch and kick him.
"I do not remember how many protesters I struck, but done (sic) so in order to prevent any further rioting and to preserve my safety," he said.
Later he gave a statement to the Independent Police Complaints Commission (IPCC) in which he said his use of force against father-of-nine Mr Tomlinson was necessary.
"In the context in which this engagement occurred, if this was me, the use of force was necessary, proportionate and reasonable in all the prevailing circumstances. It's extremely difficult now to recall this brief, almost momentary event clearly," he said.
Harwood, who joined the Met in 1995, said he had no real briefing about what was expected to happen at the protests that day.
However he understood that they were expected to be "substantial, unruly and potentially extremely violent", jurors were told.