Jurors trying a policeman accused of killing Ian Tomlinson during the G20 protests heard Pc Simon Harwood had a "rush of blood to the head".
Prosecutor Mark Dennis QC told Southwark Crown Court the officer would seek to justify his "violent assault" on the 47-year-old as necessary in the course of his duties.
He said: "We disagree... it was more akin to thuggish behaviour ".
Pc Harwood had been left "humiliated" by a failed attempt to arrest a protestor a few minutes before he struck Mr Tomlinson, the court heard.
Instead of returning to his role as a van driver he joined colleagues clearing the Royal Exchange near the Bank of England.
The passage was almost empty and there was no rioting but Pc Harwood adopted an "aggressive posture", Mr Dennis said.
He pushed over a cameraman and then floored Mr Tomlinson who winced in pain as he lay on the ground, according to eyewitnesses.
Neither Pc Harwood colleagues approached Mr Tomlinson to arrest him.
"In that case was such force justified?", Mr Dennis asked.
Pc Harwood made a statement later that night covering his actions throughout the day saying he only used "reasonable force".
When amateur video footage of him hitting Mr Tomlinson emerged one week later, the officer recognised himself and came forward.
The court heard he has declined to answers questions instead giving the Independent Police Complaints Commission prepared statements.
Pc Harwood denies manslaughter and the trial expected to last four weeks continues tomorrow when jurors will see the amateur video.