A police officer killed a newspaper seller in "a gratuitous act of aggression" while his "blood was up", a court has been told.

PC Simon Harwood, from Carshalton, is accused of hitting Ian Tomlinson with a baton and pushing him to the ground as he prepared to walk away from a line of 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 G20 protests that day - collapsed and later died.

PC Harwood maintains he used reasonable force and denies manslaughter.

Mr Tomlinson was facing away from Harwood and would have been "taken completely by surprise" when he was hit, jurors were told, and had little opportunity to protect himself from a heavy fall.

Opening the prosecution case at 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."

PC Harwood's reaction was "wholly disproportionate" in the circumstances, the court heard.

Mr Dennis continued: "There had been no need to use any force upon Tomlinson, let alone a forceful baton strike followed by a powerful push to the back that sent Ian Tomlinson flying to the ground.

"The display of force has all the hallmarks of a gratuitous act of aggression by a lone officer whose blood was up having lost the self control to be expected of a police officer in such circumstances, and who was going to stand no truck from anyone who appeared to be a protester and to be getting in his way."

Initially, pathologist Dr Freddy Patel found Mr Tomlinson had died from a heart attack, but questions were raised when an American tourist came forward with a film recording of him being hit.

Further medical reports suggested in fact he died from an injury to his liver which caused internal bleeding and led to cardiac arrest.