Simon Harwood is accused of hitting Tomlinson with a baton and pushing him to the ground as he prepared to walk away from officers in the City of London in April 2009.
Within minutes the 47-year-old — who had been trying to walk home but found his usual route blocked due to the G20 protests that day — collapsed and later died.
Harwood, from Carshalton in Surrey, maintains he used reasonable force and denies manslaughter.
Tomlinson was facing away from Harwood and would have been “taken completely by surprise” when he was hit, jurors were told.
Opening the case at Southwark Crown Court, Mark Dennis, prosecuting, 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.
Initially pathologist Dr Freddy Patel found that Tomlinson had died from a heart attack, but questions were raised when a tourist came forward with a film of him being hit.
Further medical reports suggested he died from an injury to his liver which caused internal bleeding and then cardiac arrest.
The trial continues.