A music teacher who punched a police officer in the face at a demonstration has been cleared of assault charges after a court ruled he was acting in self-defence.

Chris Bluemel said today his acquittal "speaks volumes" about the excessive force used by police at protests, after three magistrates decided he had been acting reasonably when he hit a police officer.

Bluemel, 28, from Southampton, had been attending a protest at an arms factory in Brighton on 4 June last year when police struck him with batons amid a concerted effort to clear the last remaining protesters from a car park. The protester punched a male officer, PC Dugan, who later complained his tooth had been loosened.

Dugan admitted at Brighton magistrates court that he used his baton on several protesters, although it was not clear whether he struck Bluemel.

For his part, Bluemel told how, in spite of complying with officers' instructions to leave the area by telling them "I am going to leave now", he was pushed and beaten repeatedly with batons on his back and left arm. The court was shown photographs of Bluemel's torso covered in bruises, taken days later at a naked bicycle ride.

Bluemel said that after being knocked to the floor, he jumped up and hit one of the officers in the face. He said he was angry that well-armed officers were striking out without warrant, and he wanted to deter further attack. The magistrates also took into account his Asperger's syndrome.

On Friday the magistrates ruled police had used excessive and therefore unlawful force as they tried to remove protesters from the car park, and cleared Bluemel of assault of a police officer acting in the execution of his duty. Bluemel was also cleared of the alternative charge of common assault.

Bluemel said he was relieved to be cleared as he feared he would lose his job teaching music at a sixth-form college. "The fantastic thing is that we now know from the court that police used excessive force," he said. "It also speaks volumes that they ruled I was acting reasonably in the circumstances."

Sussex police said: "We put together a thorough investigation, which was put before the court, and we respect the court's decision."