HONG KONG (AFP) - Jailed Hong Kong activist Joshua Wong will mark his 21st birthday in court on Friday (Oct 13) to face more charges over his role in pro-democracy protests as his parents say they are standing by him.

Wong and two other student leaders were jailed in August for six to eight months for their roles in the initial protest that sparked the months-long Umbrella Movement demonstrations and street blockades in 2014, convictions rights groups have called politically motivated.

His father Roger Wong said he was surprised by the jail term and shocked by the reaction of Hong Kong's legal community, with two powerful organisations releasing statements defending the judicial system and judges.

"The elites in law were congratulating each other. I find that disgusting," said Wong.

"I thought the statements would come with comprehensive legal analysis, but there was none of that... just brown-nosing," he added.

Wong, a retired IT professional, and his wife have been visiting their son in prison, and say he is putting on a brave face.

"If I was in his position, in front of my parents, I would say positive things too and not want them to worry," said Wong, 53, speaking to AFP at the family's compact apartment filled with photos, books and bible references.

"I think the biggest pain for him is not being able to go on the internet," he said.

Unlike most middle-class parents in education-obsessed Hong Kong, Wong senior said he was open-minded about his son's studies and future path - they always knew he was politically minded.

When he was small, "he would have a bottle in his mouth but still continue to speak. Of course, we didn't know what he was saying... (but) he always enjoyed expressing himself," Wong said.

"His academic record would not be good enough for Oxford or Cambridge, but they have invited him over as a speaker - it's just a different role," he said.

But he said he would still have supported his son's political career if he ran for office. Wong's prison term now bars him from running for the next five years.

As Beijing and local authorities tighten the screws on political activism in the semi-autonomous city, Wong says he is not worried about this difficult path his son has chosen - he just wants him to stay strong.

"I hope he will keep his innocent heart to do what is right, and remain happy," Wong senior said.

"Some things don't show their results immediately, but you can persist."