The Pc who pushed Ian Tomlinson at the G20 protests in London did so after a strike on the thigh had failed to move the newspaper seller away from the police line, an inquest has heard.
Simon Harwood told the hearing he had been "amazed" that Mr Tomlinson fell forward after the "poor push" in 2009.
Mr Tomlinson, 47, got back to his feet but collapsed and died minutes later.
Pc Harwood later said he was very sorry "if it is the case that in any way I have caused Mr Tomlinson's death".
'Ignoring requests'
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, on 1 April 2009.
During his evidence, he explained events leading up to the push on Mr Tomlinson and said he had palmed off protesters and pushed a cameraman as confrontations "got out of hand".
He said he had first seen Mr Tomlinson talking to two police-dog handlers and an officer near a bike shed on the Royal Exchange.
He said he saw a police dog bite Mr Tomlinson on the ankle and another officer push him away from the police line.
"I then remember Mr Tomlinson moving in front of me from my right to my left as though he was walking or encroaching into the police line," he said.
Pc Harwood said he struck Mr Tomlinson because, although he was not a threat, he was ignoring other officers' requests for him to move.
Describing the moments he hit the father-of-nine, the officer said: "I struck the upper part of his leg with my baton. I didn't get a reaction so I pushed him with the right palm of my hand to the top right of his shoulder.
Ian Tomlinson on 1 April 2009 Mr Tomlinson was filmed moments before he died on 1 April 2009
"Once I pushed him, he fell forward. I was amazed that he fell forward. I looked for other threats, then I moved back behind the police dogs."
Pc Harwood said he had used forced "initially as an encouragement to make him move away".
"I hit him in the thigh because usually that's where you get less harm but the maximum reaction. It really encourages people to move away."
He then said he had pushed Mr Tomlinson because he had got no reaction to hitting him in the thigh with the baton "and I needed him to move away from the police line".
Mr Tomlinson, who was not a protester and had been selling newspapers in Fish Street, was on his way home when he encountered the protests.
The inquest, at the International Dispute Resolution Centre, is examining the actions of the police, the pathologist and independent investigators.
'Clenched fist'
Pc Harwood told the court of the moments leading up to Mr Tomlinson's death.
He said he had had to "recompose myself" after several confrontations as the demonstrations got "out of hand".
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“Start Quote
I was starting to believe that this was getting out of control”
End Quote Pc Harwood
He said he had been "frightened and confused" in the tense atmosphere and after being hit on the back by an object and palming off a protester who had approached him with a "clenched fist", he had felt "very" scared.
As video footage was played to the 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 had to find time to recompose himself after realising he had lost his colleagues.
On Monday, Pc Harwood told how he had been in fear for his life after attracting the attention of a "hostile crowd".
He said protesters had jeered and shouted as he tried to arrest a man on suspicion of criminal damage to one of the police vans.
There was a "large gasp" from the crowd as the suspect he had hold of collided with the door of a police van, he said.
Footage showed him leading the man further away from the vans with the crowd surging behind him.
"At the time because he was becoming more aggressive, more hostile, I was starting to believe that this was getting out of control.
"I was aware there was a very hostile crowd and I was actually in fear for my life then from what was coming towards me."
The inquest continues.