Three leaders of Hong Kong's "Umbrella Revolution" avoided jail Monday over 2014 pro-democracy protests as a court said political tension would not sway its judgement, in a city divided by Beijing's tightening grip.

The young campaigners -- Joshua Wong, Nathan Law, and Alex Chow -- were charged for a protest in September 2014 which saw students climb over a fence into Hong Kong's government complex, known as Civic Square.

They were calling for fully free leadership elections for the semi-autonomous city and their arrests at the time sparked wider rallies.

Those exploded two days later when police fired tear gas on the crowds, triggering mass demonstrations that brought parts of Hong Kong to a standstill for more than two months.

The umbrellas protesters used to defend themselves from pepper spray and tear gas gave the movement its name.

Tensions have remained high since the rallies ended without concessions from Beijing on political reform, splitting society into those who want to fight for greater autonomy and those who think there is little to gain.

Magistrate June Cheung said it would be unfair if she were influenced by the current political atmosphere into handing down a "deterrent sentence".

"The court believes the case is different from an ordinary criminal case. I accept they were genuinely expressing their views," she said in sentencing at Eastern Magistrates' Court.

Cheung added the three had no prior convictions, were concerned about social issues and passionate about politics.

"They turned it into action," she said.

Wong, 19, and Chow, 25, had been charged with taking part in an unlawful assembly at Civic Square, while Law, 23, was charged with inciting others to take part.

They were facing possible two-year jail terms.

Wong and Law were given community service.

Chow received a three-week sentence, suspended for a year -- he could not complete community service because he would be studying in the UK. He will not serve jail time unless he offends in the coming year.

The defendants praised Cheung for her leniency.

"The court has taken the view that the Umbrella Movement and entering Civic Square was not for personal gain but public good," Wong said.

Law added it showed the three had been acting for "justice, benefits of society and people's civil liberties".

"She sent a message that such rights should be respected," Law said.

- 'Violation of rights' -

However, Human Rights Watch Monday slammed the authorities for pursuing the case at all, saying it was a "violation of their rights to peaceful expression and assembly".

Their convictions last month -- in the highest profile court case to emerge from the pro-democracy movement -- were also slammed by rights group Amnesty International, which described it as a "chilling warning" to activists.

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

Both were acquitted in June over a separate anti-China rally in the summer of 2014.

Since the failure of the Umbrella Movement to achieve political reform, an increasing number of young activists are calling for a break from the mainland -- a message that has infuriated Beijing.

Wong and Law have recently set up a new political party, Demosisto, which is campaigning for self-determination for Hong Kong. Law will run for lawmaker in upcoming elections.

Hong Kong was returned to China by Britain in 1997 with its freedoms guaranteed for 50 years.

But there are growing concerns that Beijing is no longer adhering to the agreement with fears China is interfering in a number of areas -- from politics to education and the media.