China on Friday rejected international criticism of the jailing of three prominent Hong Kong activists, warning against using "so-called democracy" to conduct "illegal violent activities".

Joshua Wong and two other young leaders of Hong Kong's huge Umbrella Movement protests were sentenced to months in jail on Thursday for their role in the 2014 rallies.

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

"Hong Kong people are fully entitled to rights and freedoms. But no one can use the excuse of so-called democracy and freedom to conduct illegal violent activities," foreign ministry spokeswoman Hua Chunying said.

"I want to reiterate that Hong Kong is a special administration of China... China is firmly opposed to any external forces interfering in Hong Kong affairs," Hua said at a regular press briefing.

Britain, Hong Kong's former colonial ruler, said it hoped the sentencing would not discourage "legitimate protest" in future.

US Senator Marco Rubio, chairman of the Congressional Executive Commission on China, described the three as "pro-democracy champions worthy of admiration, not criminals deserving jail time".

Amnesty International slammed authorities' pursuit of jail terms as a "vindictive attack on freedom of expression and peaceful assembly".

Wong, who became the face of the mass protests while still a teenager, as well as Nathan Law and Alex Chow were given terms of six months, eight months and seven months respectively after the court upped their previous non-custodial sentences.

Anyone who receives a jail term of more than three months is barred from running for Hong Kong's partially directly elected parliament for five years.

Defence lawyers argued the trio had insisted on non-violence including at Civic Square, where there was pushing and shoving between protesters and police.

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.

Source: AFP