A photographer was on Friday acquitted of aiding and abetting public disorder in relation to a pro-democracy protest in Central last year in which a mainland man was assaulted by a crowd of people.

Prosecutors said the assault victim, an employee of JPMorgan Chase, had tried to go into a building to escape an angry crowd he had been arguing with on October 4. But Marc Progin, 75, closed the door on him.

Eastern Court ruled that while Progin's behaviour was suspicious, it was possible he had only been trying to find the best position to photograph the scene.

Magistrate Stephanie Tsui said security camera footage showed Progin moving away from the victim, and the court also accepted that he did not know anyone among the protesters.

Speaking to journalists after the hearing, Progin, a Swiss citizen who lives in Hong Kong, said he would not be deterred from covering protests in future.

"I was at the wrong place, at the wrong moment. And then it turned out somebody was attacked, and I was arrested for the reason that you have heard," he said.

"Frankly speaking, I was just doing what you [reporters] are doing at the moment ... that means there was nothing wrong in doing that. So I think the court was very fair, and I am very happy."