A policeman struck a G20 protester twice with a baton after mistaking a carton of orange juice in her hand for a weapon, a court has heard.
Metropolitan Police sergeant Delroy Smellie, 47, said he hit Nicole Fisher, 36, because she posed a threat during the protests on 2 April last year.
The officer went on trial on Monday accused of common assault by beating against the animal rights activist.
Video footage shown in court revealed he pushed and struck Ms Fisher.
Prosecutor Nick Paul said when she remonstrated with Sgt Smellie, shouting that he had hit a woman, he pulled out his baton, known as an asp, and hit her twice on the leg.
Perceived as threat
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 Sgt Smellie's actions could not be justified.
Mr Paul said Sgt Smellie pulled out his baton after noticing Ms Fisher holding an object in either hand.
He 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."
Ms Fisher had in fact been carrying an open carton or orange juice, which sprayed over Sgt Smellie when he hit her, and a camera.
The court head Ms Fisher, of Brighton, was depressed and "unwilling" to attend proceedings over fears the defence would focus on her lifestyle and background.
District Judge Daphne Wickham ruled her statement to independent investigators could not be admitted in evidence, but turned down an application by the defence to throw out the case.
The trial went ahead without her. The case is continuing.