A Swiss photographer who closed a door on a Chinese mainlander moments before he was assaulted during last year's Hong Kong protests was on Friday found not guilty of aiding the attack.

Marc Progin was cleared of "aiding and abetting public disorder" over the incident in which JP Morgan employee Lin Nan was punched, Magistrate Stephanie Tsui said at the city's Eastern Magistrates' Courts.

Footage of Progin closing the door leading to JP Morgan's regional headquarters moments before a masked man assaulted Lin went viral and caused widespread anger in mainland China.

Mandarin-speaking Chinese mainlanders sometimes face hostility and abuse from Cantonese-speaking Hong Kongers, a trend that has increased as Beijing ramps up control of the financial hub.

Prosecutors said Progin, 75, deliberately shut the door as an argument broke out between Lin and a crowd of pro-democracy supporters last October, effectively enabling the assault on Lin.

When protests swept through Hong Kong last year, Swiss photographer Marc Progin often picked up his camera to document the crisis
When protests swept through Hong Kong last year, Swiss photographer Marc Progin often picked up his camera to document the crisis AFP / Philip FONG

Defence lawyers said Progin was simply doing his job and that he closed the door to get a better angle to capture the argument.

The magistrate said the court could not rule out the possibility that he was there "to take good pictures" and that there was no evidence he was in contact with anyone in the crowd.

Progin gave a thumbs up as he left the court and said he was "very happy" with the verdict.

He had faced up to a year in jail if found guilty.

Lin's attacker has yet to be arrested.

Swiss photographer Marc Progin, 75, had been charged with 'aiding and abetting public disorder'
Swiss photographer Marc Progin, 75, had been charged with 'aiding and abetting public disorder' AFP / Anthony WALLACE

Lin had testified that he felt frightened as the crowd shouted at him and that he did not believe Progin was there to take photographs.

But the defence argued that Lin's decision to stop, turn around and say: "We are all Chinese" had provoked the protestors.

Photographs taken by Progin during the incident were submitted as evidence that he was a working professional and not a member of the protest crowd.

Progin moved to Hong Kong in 1976 and forged a career as a watchmaker. After retirement he became an adventurer and photographer with multiple exhibitions.

When protests swept through Hong Kong last year he often picked up his camera to document the crisis.

Millions peacefully took to the streets as anger towards Beijing's rule exploded, with clashes between more militant protesters and police becoming a near-daily occurrence.

Street fights also routinely broke out between people on opposing sides of the political divide.

Authorities have arrested more than 10,000 people and brought multiple prosecutions against leading pro-democracy figures.

Chinese authorities imposed a draconian national security law on Hong Kong in late June, describing it as a "sword" to hang over the heads of its critics.

In the latest blow to the city's beleaguered democracy movement, 15 lawmakers this week said they would resign from the city's legislature in protest at a Beijing-sanctioned ousting of four colleagues.

China