Footage of demonstration in City of London shown at packed court, where sergeant says he 'acted in self-defence'

A police sergeant twice struck a female G20 protester with a metal baton in what he said was "self-defence" after mistaking a carton of orange juice in her hand for a weapon, a court heard today.

Delroy Smellie, 47, a sergeant in the Metropolitan police, said he lashed out at Nicola Fisher, 36, in a "pre-emptive strike" during a confrontation outside the Bank of England on 2 April last year.

Smellie, a member of the Met's elite Territorial Support Group (TSG), went on trial accused of common assault by beating. He denies the charge, and his lawyers said they would argue he was seeking to defend himself and his colleagues.

The trial opened at a packed City of Westminster magistrates court and was shown video footage of the moment he slapped Fisher across the face before drawing his baton and striking her twice. Smellie showed no emotion as the footage played. The incident occurred during a memorial vigil for Ian Tomlinson, the newspaper vendor who had died the previous day after being attacked by another Met officer, also from the TSG.

Opening the case, Nicholas Paul, prosecuting, said Smellie had "lost his self control" during an "excessive and unjustified" attack on Fisher. "He went from level one to level five without considering the intervening steps," said Paul.

Paul showed district judge Daphne Wickham, who is trying the case without a jury, CCTV footage of the police operation and amateur footage of the clash between Smellie and Fisher that was posted on YouTube and handed to the Guardian.

He described the video as the core of the case against Smellie. The court also heard extracts of two interviews between Smellie and officials from the Independent Police Complaints Commission (IPCC).

The judge heard how Smellie told investigators that, after back-handing Fisher, he reached for his baton after noticing Fisher was holding an object.

The prosecutor said: "She had weapons in her hand and he perceived her as a threat." Fisher was in fact carrying an open carton of orange juice, which sprayed over Smellie when he hit her, the court heard.

The prosecutor said Fisher may have been aggressive, but Smellie's actions could not be justified. "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 … not in any way [was she] seeking to get involved in an attack.

Footage shown to the court started from around 2:30pm on 1 April, when TSG officers arrived near Royal Exchange to replace officers from City of London police, who had formed a cordon around protesters.

Paul said the "attitude and atmosphere" created by police changed upon the arrival of the TSG.

"There had been reports of people masking up and the unit to which Sgt Smellie was in charge was briefed to contain people within a cordon to prevent a breach of the peace and any other disorder such as had happened the day before," Paul said. "It was these instructions that informed Sgt Smellie's approach to the demonstrators that they dealt with."

The footage showed how moments before the alleged assault a demonstrator attempting to leave the cordon was pushed back by police, prompting an angry response from the crowd.

Fisher approached Smellie before being pushed away. As she moved back towards the officer, the footage showed him strike her with the back of his hand in what the court heard was a legal "clearance-swipe". But as Fisher began shouting "you hit a fucking woman" and moving towards him again, he took out his baton and hit her twice on the legs. The prosecutor said it was these final two blows that were at the centre of the case.

Earlier in proceedings, Wickham refused an application from the defence to get the case thrown out after the alleged victim – Fisher – did not turn up for the first day of the hearing.

Fisher, who had been due to give evidence, was described by the judge in pre-trial argument as having recently been "nervous, weary [and] unsure of whether the CPS [Crown Prosecution Service] is part of the police".

Paul also indicated she may be fearful of the press coverage the trial would receive. From the defence, the court also heard Fisher may be suffering from depression.

While allowing the trial to proceed, Wickham refused, in the absence of Fisher, to accept as evidence a statement she provided to the IPCC on 16 April. The court heard Fisher, from Brighton, had been represented by Max Clifford and sold her story to a national newspaper.

A witness, photography student Beth Wilson, told the court she saw him "use his baton with force". "She didn't appear to be a physical threat to him," she said.

The trial is expected to last four days.