A police officer shoved Ian Tomlinson to the ground at the G20 protests minutes after palming off several protesters and pushing a cameraman over, an inquest has heard.
Pc Simon Harwood said he had to "recompose myself" after several confrontations as the demonstrations got "out of hand".
He said he had been "frightened and confused" in the tense atmosphere before the newspaper seller's death. After being hit on the back by an object and palming off a protester who had approached him with a "clenched fist", the officer said he had felt "very" scared.
When asked about his conduct during another confrontation, he told jurors: "I thought it was proportionate and reasonable to do so."
As video footage was played to a jury of Pc Harwood at the Royal Exchange Buildings, the officer said: "I was frightened and confused, I wondered where I was."
He said he had to find time to recompose himself after realising he had lost his colleagues.
Pc Harwood, part of the Metropolitan Police's specialist Territorial Support Group (TSG), was caught on film pushing Mr Tomlinson to the ground in Cornhill, central London, during the protests two years ago.
The footage shows Mr Tomlinson turning away from a line of officers with his hands in his pockets before he is shoved in the back. The 47-year-old was helped to his feet but collapsed and died about 100 yards away shortly afterwards.
Devastated family members stormed out of the inquest, at the International Dispute Resolution Centre in London's Fleet Street, on Monday, after Pc Harwood said he wanted to "help" them.
Judge Peter Thornton has warned him he is not obliged to answer any questions which might incriminate him.