Jailed Hong Kong media tycoon Jimmy Lai is among eight democracy activists who have been handed new prison sentences for attending protests on the 70th anniversary of the founding of communist China that were followed by a sweeping crackdown.

Lai, who is already behind bars for taking part in earlier protests, must now serve a total of 20 months after pleading guilty to organising an unlawful assembly on 1 October 2019.

Seven other leading activists, including 25-year-old youth campaigner Figo Chan, as well as former politicians Lee Cheuk-yan and Leung Kwok-hung, were also given new jail sentences.

Many flashed "victory" hand signals on their way to court in a police van.

The new sentences are the latest in a relentless and successful campaign by China to smother dissent and dismantle Hong Kong's democracy movement.

Hong Kong was convulsed by months of huge and often violent pro-democracy protests in 2019 in the most serious challenge to Beijing's rule since the city's 1997 handover.

The clashes with police on China's 1 October National Day were some of the worst of that period.

It was a vivid and embarrassing illustration of how huge swathes of Hong Kong's population seethe under Beijing's rule as the government celebrated 70 years since communist China's founding.

While President Xi Jinping oversaw a huge military parade in Beijing, clashes between hardcore protests and police raged across Hong Kong that day.

The march attended by the activists who were jailed remained largely peaceful, but it did not have official police permission, a requirement in Hong Kong.

"It was naive to believe a rallying call for peaceful and rational behaviour would be enough to ensure no violence," district judge Amanda Woodcock said as she handed down jail sentences to the eight activists.

China has responded to the democracy rallies with a broad clampdown on Hong Kong, including the imposition of a sweeping national security law that outlaws much dissent.

Yesterday Hong Kong authorities banned the annual 4 June vigil marking Beijing's 1989 Tianamen Square crackdown, with security minister John Lee warning the security law could be used against those who defy the ban.

More than 10,000 people were arrested during Hong Kong's democracy protests, with around 2,500 convicted for various offences.

Most of the city's prominent democracy leaders are either under arrest, in jail or have fled overseas.

More than 100 people, including Lai, have been charged under the security law, which carries up to life in jail.

Those handed jail terms are from the more moderate wing of Hong Kong's democracy movement. Four were already serving jail sentences for taking part in protests.

Many of them have spent decades advocating non-violence in their ultimately fruitless campaign for universal suffrage.

Figo Chan, for example, was a key figure in the Civil Human Rights Front, the coalition that organised some of the largest rallies of 2019 when hundreds of thousands turned up.

Supporters chanted "Add oil!" - a Chinese phrase of encouragement - as the group were led into court.

At a mitigation hearing earlier in the week, Chan accused Hong Kong's unelected leaders of failing to give citizens an avenue to voice their dissatisfaction.

"If the government listened to people's demands, then it would not be necessary for the people to use violence to get the government to respond," he told the court.

Lee Cheuk-yan, 63, said he had no regrets about the prospect of going to jail.

"For over 40 years I have strived for democratic reform in China," he told the court.

"This is my unrequited love, the love for my country with such a heavy heart."

China says the clampdown and security law is needed to return stability.

It has dismissed the democracy demands and says the protests were instigated by "foreign forces" who want to undermine China.

Many Western nations say China has shredded its promise that Hong Kong could maintain certain freedoms and autonomy under a "One Country, Two Systems" arrangement agreed before the city's 1997 handover.