Three young Hong Kong activists who led the city's massive "Umbrella Revolution" pro-democracy protests walked free from court Wednesday after a renewed bid by prosecutors to jail them.

It comes as tensions remain high in the semi-autonomous city with fears growing that China is tightening its grip, sparking a growing independence movement.

Nathan Law, 23, who was recently voted in as the city's youngest lawmaker and wants self-determination for Hong Kong, was one of the trio in court and slammed Wednesday's hearing as a political move.

"The Department of Justice and the government are basically hunting those (who took part in) civil disobedience and those who uphold democratic values," he told reporters.

Law, Joshua Wong and Alex Chow were all convicted in August for taking part in, or inciting others to take part in, a protest that led up to the major pro-democracy demonstrations of 2014 calling for fully free elections of the city's leader.

They were given community service or suspended sentences after the magistrate said she believed they had been "genuinely expressing their views" during the protest, which saw students climb over a fence into the Hong Kong government complex.

But prosecutors sought a sentence review in court Wednesday, asking for jail terms for the three.

They argued the protest they led was large-scale and that the defendants had not shown genuine remorse.

Magistrate June Cheung, who also handed down the original verdict and sentencing, dismissed the review case.

"During sentencing, I already took into account carefully the seriousness of the case, thus imposing community service as well as a suspended sentence -- which can legally replace jail terms -- to the three, who are first offenders," Cheung said.

It is not clear whether prosecutors will try to appeal further.

Law said prosecutors must have come under "huge political pressure" to bring the review.

Chow, 25, said it had been brought "to suppress those who oppose the government and ask for universal suffrage through the legal system".

Wong, 19, who became the face of the Umbrella Movement, urged supporters to continue civil disobedience as a way to campaign for freedoms and rights.

Hong Kong has been governed under a "one country, two systems" deal since it was handed back by Britain to China in 1997.

The agreement is designed to protect Hong Kong's freedoms for 50 years, but there are concerns those liberties are already being eroded.

Law and Wong have together set up a new political party calling for Hong Kongers to have a choice over their sovereignty when the handover deal expires in 2047.

Wong is still too young to stand for parliament as the minimum age for candidates is 21.