A policeman has been acquitted of killing Ian Tomlinson during G20 protests in London by striking the 47-year-old bystander with a baton and pushing him to the ground as he walked away from police lines.

The jury at Southwark crown court on Thursday cleared PC Simon Harwood, 45, a member of the Metropolitan police's elite public order unit, the Territorial Support Group, of manslaughter following one of the most high-profile cases of alleged police misconduct in recent years.

Harwood told the court that while in retrospect he "got it wrong" in seeing Tomlinson as a potentially threatening obstruction as police cleared a pedestrian passageway in the City on the evening of 1 April 2009, his actions were justifiable within the context of the widespread disorder of that day.

Speaking outside the court, the Tomlinson family said: "It's not the end, we are not giving up for justice for Ian." They said they would now pursue a civil case.

The jury's verdict, after four days of deliberations, brings about something of a legal contradiction: 14 months ago another jury, at the inquest into Tomlinson's death, ruled that he was unlawfully killed by Harwood. The inquest ruling was made on the same standard of proof as a criminal trial, that is, beyond reasonable doubt.

Neither jury heard details of Harwood's prior disciplinary record, which can only be reported now. This includes how he quit the Met on health grounds in 2001 shortly before a planned disciplinary hearing into claims he illegally tried to arrest a driver after a road rage incident while off duty, altering his notes to retrospectively justify the actions. Harwood was nonetheless able to join another force, Surrey, before returning to serve with the Met in 2005.

He allegedly punched, throttled, kneed or threatened other suspects while in uniform in other alleged incidents.

The verdict will come as a huge disappointment to Tomlinson's family, following a saga that began when the father of four, who was stepfather to his wife's five other children, collapsed as he tried to make his way home through police lines. It followed a day of protests connected to the meeting in London of leaders from the G20 group of nations. He died shortly afterwards.

Tomlinson had been an alcoholic for some years and was living in a homeless hostel. It was initially presumed he died from natural causes, a conclusion supported by an initial postmortem examination, which gave the cause as heart failure.

But six days later the Guardian published video footage, shot by an American in London on business, which showed a policeman in riot gear striking Tomlinson on the leg with a baton before shoving him violently to the pavement, minutes before his final collapse.

Three pathologists involved in two further postmortem examinations said Tomlinson instead died from internal bleeding associated with his liver and consistent with being pushed to the ground. While the officer was soon identified as Harwood, prosecutors initially decided against charging him, changing their mind only after the inquest verdict.

The trial hinged on two key questions: firstly, whether Harwood's actions amounted to a criminal assault; then, whether they directly led to Tomlinson's death.

The first issue was simple, the prosecution argued: Harwood carried out "a gratuitous act of aggression", Mark Dennis QC told the jury. Harwood had recklessly abandoned the police van he was designated to drive to arrest a man seen writing graffiti on another vehicle. Humiliated when the man wriggled free, he opted to join a line of other officers clearing a pedestrian passageway by the Royal Exchange complex.

But in his evidence Harwood said he had been separated from his van by a threatening crowd before following orders to clear the passage. He insisted his actions towards Tomlinson were correct at the time, a version of events supported by two other officers at the scene called as defence witnesses.

The issue of cause of death saw the testimony of the first pathologist, Dr Freddy Patel, who reasserted his belief that Tomlinson died from heart failure, placed against that of Dr Nat Cary, who told the court that even a relatively small amount of internal bleeding would have caused death. The jury was not told that Patel has twice been suspended by medical authorities for mistakes in other postmortem examinations and is no longer on the Home Office's register of approved pathologists.

No police officer has been convicted for manslaughter for a crime committed while on duty since 1986.