A police officer killed newspaper seller Ian Tomlinson in a gratuitous act of aggression "akin to thuggish behaviour", a court has been told.
Pc Simon Harwood is accused of striking Mr Tomlinson with a baton and pushing him to the ground as he prepared to walk away from a line of officers policing the G20 protests in London.
The 47-year-old - who had been trying to head home but found his usual route blocked - collapsed within minutes and later died.
Harwood, 45, from Carshalton in Surrey, maintains that he used reasonable force and denies manslaughter.
But Mr Tomlinson was facing away from Harwood and would have been "taken completely by surprise" when he was hit in the City of London on April 1 2009, jurors were told. He had little opportunity to protect himself from a heavy fall, they heard.
The incident came not long after Harwood had been "embarrassed if not humiliated" by his failure to arrest a demonstrator he spotted trying to graffiti a police van, said prosecutors.
His "blood was up" as a result, and the scene was set for his encounter with Mr Tomlinson, they argued.
Opening the case against Harwood at London's Southwark Crown Court, Mark Dennis QC said: "The assault upon Ian Tomlinson had been an unnecessary and unreasonable use of force by the defendant.
"Ian Tomlinson was not posing any threat to the defendant or any other police officer. He was displaying no aggression towards anyone nor even making any provocative comments."
Harwood's reaction was "wholly disproportionate" in the circumstances, the court was told.