A Metropolitan police sergeant who was filmed hitting a woman with a baton at the G20 demonstrations was cleared today after a judge ruled he acted lawfully.

Delroy Smellie was suspended last year after video footage was posted on YouTube showing him back-handing a protester and striking her twice on the legs with his metal baton. He was acquitted of assault by beating after a four-day trial in which his alleged victim, protester Nicola Fisher, declined to give evidence.

Smellie, from the Met's territorial support group (TSG), a specialist public order unit, argued during his trial that he believed Fisher posed a threat to himself and fellow officers. He said he repeatedly struck Fisher after mistaking a carton of orange juice and digital camera she was carrying for weapons.

The district judge, Daphne Wickham, said there was no evidence that his use of the baton was not approved, correct or measured, adding that Smellie had a "mere seven seconds" to act, and other witnesses had feared for his safety.

She said: "It was for the prosecution to prove this defendant was not acting in lawful self-defence. I have found the prosecution has failed in this respect and the defendant has raised the issue of lawful self-defence and as such is entitled to be acquitted."

Nodding to the judge after the hearing, Smellie said: "Thank you very much." The 47-year-old sergeant refused to talk to reporters after saying: "I don't think so, I've got a reputation to protect."

His acquittal is a setback for the Independent Police Complaints Commission (IPCC), which received almost 300 complaints about police behaviour at the G20 protests. Despite numerous IPCC investigations, no officer has faced serious disciplinary action and none have been successfully prosecuted.

Its inquiry was a response to the worldwide attention sparked when amateur footage of the incident was posted on YouTube and handed to the Guardian. The IPCC justified its investigation today, saying the public was "understandably concerned" over the footage, pointing out that Smellie could still face disciplinary proceedings. The Met said in a statement that the officer had been returned to duties. Fisher, 36, suffered bruises to her leg and enlisted the publicist Max Clifford to sell her story to a national newspaper for around £26,000. She failed to attend the trial, saying she was depressed.

Clifford said his client would be disappointed but not surprised by the outcome. "She sees it as a total miscarriage of justice. She was convinced that she wouldn't get justice," he said.

The incident took place near the Bank of England, on 2 April, during a memorial vigil for Ian Tomlinson, the newspaper vendor who collapsed and died the previous day after being attacked by another officer from the TSG. Bystanders told the court they had seen Fisher acting in an aggressive way.

Wickham watched video footage of the incident and looked at dozens of photographs. They showed how Smellie first swiped Fisher with the back of his hand. The prosecution contended that while that manoeuvre was lawful, his repeated use of a baton against her legs showed the officer had lost his self-control.

Defending the force he used against Fisher, Smellie suggested the images shot by bystanders did not convey the threat he felt she posed.

"Not one photograph or piece of footage comes close to reflecting the fear as I turned around to see this crowd and its proximity, both to myself and my officers," he told the court. "At the time I thought, this is it. She is deliberately coming from a blind spot. The reason she is coming from a blind spot is to hide her intention so she can approach and attack her target – me." Smellie told the court that, had he used other tactics, such as an elbow, he could have broken Fisher's bones.

Talking about the back-handed strike, he said: "Does it really need a broken jaw, which could easily have happened if I struck her with my left elbow in her face? I thought that the most reasonable level of force would be a flick with the hand as a distraction clearance."

Smellie said that he chose to strike her legs with a baton – rather than her arm – for a similar reason. "The force of the strike, the differential in size – I could easily have snapped that arm." He said that, after striking her: "I hoped that she would either fall to the ground, drop the weapons or go away and get back … But certainly to ensure that she was not able to use those weapons or that the weapons were not able to be used."

In her ruling today, the judge said she was satisfied that, faced with a protester "seeking confrontation", Smellie could not have used other options.

At 10.45am today, Ian Tomlinson's family will gather for a memorial on the first anniversary of his death at the scene of his death. The family have released a letter to the DPP, calling on him to decide whether the officer accused of assaulting Tomlinson should be charged with manslaughter.