Hong Kong democracy campaigners raised fears Friday that they too would be jailed after the city's Beijing-backed government succeeded in putting Joshua Wong and two other prominent activists behind bars. 

Supporters and rights group said the sentencing Thursday was more proof that Beijing is tightening its grip on the semi-autonomous city and that rule of law is being compromised.

Hong Kong enjoys freedoms unseen on the mainland after being handed back to China in 1997 under a "one country, two systems" deal, but there are growing fears those rights are disappearing.

Wong, Nathan Law and Alex Chow were handed sentences of six to eight months by the Court of Appeal for their role in 2014's massive Umbrella Movement protests, which called for fully free leadership elections and were an unprecedented challenge to Beijing.

The court overturned previous non-custodial sentences saying they were too light and did not serve as a deterrent to activists who were undermining stability in Hong Kong.

Nine more democracy campaigners -- including the three "Occupy Central" founders who first called for residents to protest on the streets in 2014 -- are due to appear in court in September over their part in the Umbrella Movement. 

All face public nuisance charges which carry maximum sentences of seven years. 

Occupy founder Benny Tai, 53, who is among the nine, told AFP that he was now bracing for a jail term.

"Personally, I am prepared in my own case -- it's likely that there will be imprisonment," Tai said.

Tai added that the court had ignored the principle of civil disobedience.

"It sets a kind of guiding principle for lower courts to follow in dealing with cases of this kind," Tai said.

- 'Unhealthy trend' -

Britain said it hoped the sentencing would not discourage "legitimate protest" in future. US politicians Marco Rubio and Nancy Pelosi condemned the judgement. 

But Wally Yeung, one of the panel of three judges that handed down the sentences, said in a written judgement that there had been an "unhealthy trend" of people in Hong Kong breaking the law for the sake of their ideals and having what he described as "arrogant and self-righteous ideas".

"Acts that undermine the public order will cause society to sink into a chaotic state and will have a seriously negative impact on the progress and development of society," he said.

The justice ministry, which brought the re-sentencing bid, said the judgement could "provide guidance to future cases of similar nature", but insisted there was no political motive.

Wong, 20, spent the first night of his six-month sentence in Pik Uk high security prison for young male offenders Thursday. Law and Chow are at another maximum security holding centre. 

Wong's mother Grace visited him Friday afternoon along with two other activists.  

In an emotional letter penned to her son before his sentencing and displayed on his party's Facebook page, she wrote: "My dear son Joshua, why has Hong Kong fallen like this? To treat the children of this generation like this?"

Supporters of the three activists will protest Friday evening outside Lai Chi Kok Prison where Law and Chow are held.