Hong Kong (AFP) - A key figure in Hong Kong's 'Umbrella Revolution' was convicted Thursday of participating in a protest that sparked mass pro-democracy rallies, in a prosecution blasted as a "chilling warning" by rights campaigners.

Joshua Wong -- who led the demonstrations that paralysed the city for months -- could be jailed for up to two years following the verdict, which comes as tensions remain high in the semi-autonomous city with fears growing that Beijing is tightening its grip.

The 19-year-old has always said the various protest-related cases against him were political persecution. Rights group Amnesty International described cases against peaceful protesters as intimidation Thursday in the wake of the verdict.

Wong was convicted for taking part in an unlawful assembly after he and others climbed over a fence into a government complex forecourt known as Civic Square on September 26, 2014, triggering wider rallies that exploded two days later when police fired tear gas to disperse crowds.

Fellow student leaders Alex Chow and Nathan Law were also convicted over the same protest Thursday -- Chow for taking part and Law for inciting others to do so.

"No matter what is the penalty... we will still continue to fight against suppression from the government," Wong said after the ruling.

"We know facing the largest communist regime in the world is a long-term battle for us to fight for democracy."

The three defendants, who smiled in resignation at the verdict, were released on bail and are due back in court on August 15 for sentencing.

The charge of participating in an unlawful assembly has a maximum sentence of up to five years, but the magistrates court where the trio were tried can only give a maximum jail term of two years per offence due to its status as a lower court.

Political analyst Ivan Choy said the public may have an "antagonistic attitude" towards the government if the trio receive a heavy sentence, while Amnesty said that "vague charges" against student leaders "smacked of political payback".

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"The Hong Kong authorities' prosecution of three pro-democracy student leaders sends a chilling warning for freedom of expression and peaceful assembly in the city," Amnesty said in a statement.

It added the city's public order laws failed to meet international standards and were being used "in an attempt to intimidate people from exercising their right to peaceful assembly".

- Interference by Beijing -

Wong was at the forefront of the 'Umbrella Movement', which brought parts of Hong Kong to a standstill for more than two months in 2014 as residents called on Beijing to allow fully free elections of future leaders.

Young campaigners were left angry and frustrated after the rallies failed to win political reform, with Wong and Law since founding a new political party, Demosisto, campaigning for self-determination for Hong Kong.

Law is also a candidate for the city's upcoming legislative council election, but he will not be able to stand if his prison sentence is over three months.

Wong has been in and out of court hearings for the past year after being charged with multiple offences linked to various protest actions.

In the ruling Thursday, magistrate June Cheung said Wong had known that climbing over the fence was "disturbing order".

Defence lawyers had argued authorities should not have fenced off Civic Square -- previously a popular protest site open to the public -- in the febrile months before the Umbrella Movement.

The prosecution said the fact they climbed into the square was unlawful and that the protest was pre-planned.

Both Wong and Law were acquitted in June over an anti-China protest in the first of a series of cases against him to reach a verdict.

Another student leader, Billy Fung, was charged Thursday over a protest in January where students stormed into an official meeting at Hong Kong University angered by the appointment of a pro-Beijing figure to a senior university role.

Hong Kong was returned to China by Britain in 1997 with its freedoms guaranteed for 50 years, but there are fears those liberties are disappearing.