UPDATE: A policeman struck a Brighton G20 protester twice with a metal baton after mistaking a carton of orange juice in her hand for a weapon, a court heard today.
Metropolitan Police Sergeant Delroy Smellie, 47, said he lashed out in a "pre-emptive strike" during a confrontation outside the Bank of England on April 2 last year.
City of Westminster Magistrates' Court heard he told independent investigators he hit Nicola Fisher, 36, because he believed she posed a threat to himself and colleagues.
The officer, a top-level trained member of the force's territorial support group, went on trial today accused of common assault by beating against the animal rights activist.
Prosecutor Nick Paul said Smellie had been on duty for about 28 hours with only a three-hour break when he received an urgent call to join colleagues in the City.
Opening the case, he said video footage of the clash revealed Smellie pushed Ms Fisher and struck her with the back of his hand, knocking off her sunglasses, after she shouted at him.
Mr Paul said when she remonstrated with him, shouting that he had hit a woman, he pulled out his baton, known as an asp, and hit her twice on the leg.
The prosecutor said it was these final two blows that were at the centre of the case as prosecutors accepted his earlier actions were reasonable and therefore lawful.
The prosecutor said Ms Fisher may have been irritating, vocal and provocative, but Smellie's actions could not be justified.
He said: "Even if her behaviour was irritating and provocative when she was standing in front of Sergeant Smellie, it is plain from the footage and photographs that there were a large number of people watching, not in any way seeking to get involved in an attack.
"It may be that her demeanour provoked Sergeant Smellie to lose his self-control and rather than responding in a way which would have involved ignoring her, warding her off or racking his asp with a view to presenting it to her, he went, if you like, from level one to level five without considering intervening steps.
"As a result of which his actions were excessive and, we say, unjustified."
The court heard Smellie arrived outside the Bank of England shortly after 2pm to replace a line of City of London Police officers who complained of being hit by missiles.
They were holding a vocal group of demonstrators inside a cordon, sometimes known as a kettle, amid fears by senior officers that disorder could break out.
The protesters had gathered around an impromptu shrine to Ian Tomlinson, a bystander who died after clashing with an officer the previous evening.
Smellie, who was dressed in a fluorescent vest and body armour, was responsible for a team of up to 10 officers trained to the highest standard for public order incidents.
Mr Paul showed District Judge Daphne Wickham, who is trying the case without a jury, CCTV of the police operation and amateur footage of the clash between Smellie and Ms Fisher.
Extracts of two interviews between Smellie and officials from the Independent Police Complaints Commission (IPCC) were also read out to the court.
Mr Paul said Smellie told investigators he reached for his baton after noticing Ms Fisher was holding an object in either hand after pushing her from the police line.
The prosecutor said: "She had weapons in her hand and he perceived her as a threat.
"The level of force used was a pre-emptive strike and was used to protect himself and his officers. It was under common law and proportionate."
The court heard Ms Fisher was in fact carrying an open carton of orange juice, which sprayed over Smellie when he hit her, and a camera.
Ms Fisher later went to a minor injuries clinic where a nurse examined a five-inch bruise on her left thigh and recommended she put ice on it and take painkillers.
Mr Paul added: "This case is not about the general policing methods adopted during the weekend of the G20 protests.
"This case is clearly about what Sergeant Smellie did in that moment when, he says, he was concerned with what he considered to be an immediate threat."
The court heard Smellie worked from 10am on April 1 until 2am the next day and then went back on duty at 5am, when he was originally posted to the Excel centre in Docklands.
Earlier today, the judge heard Ms Fisher was depressed and "unwilling" to attend proceedings over fears the defence would focus on her lifestyle and background.
The court head Ms Fisher, of Brighton, has been represented by Max Clifford, sold her story to a national newspaper for £26,000 and gave evidence to the Home Affairs Committee.
Ms Wickham ruled her statement to independent investigators could not be admitted in evidence but turned down an application to throw out the case by the defence.
Ms Fisher's former boyfriend Gavin Shepherd, who was also due to be a witness, also failed to attend court.
Smellie gave his address in court as Larkhall Lane, Clapham, one of the Metropolitan Police's territorial support group bases.
The trial, which is expected to last up to four days, was adjourned until tomorrow.