Thousands of people took to the streets of Hong Kong on Sunday to mark China's national day with a pro-democracy rally, and voice growing fears that the city's liberties are under threat from Beijing.

Emotions have been running high in the semi-autonomous city since protests in 2014 calling for Beijing to grant fully free leadership elections.

But the mass demonstrations, which blocked thoroughfares for 79 days and brought parts of the city to a standstill, failed to achieve political reform.

Sunday's protest, dubbed an “anti-authoritarian rally”, follows the recent arrests of prominent pro-democracy activists, including a former lawmaker, that have renewed anti-China sentiment.

A number of other activists, including founding members of the 2014 campaign known as the Umbrella Movement, are also facing charges and possible jail terms.

“Authoritarian rule has already become Hong Kong's reality,” Benny Tai, an organiser of the 2014 demonstrations, told protesters, who were mostly dressed in black.

“We are having today's rally ... because we hope more Hong Kong people will see the true nature of the government,” Tai, a law professor, said.

Participants in Sunday's protest singled out the city's leader Carrie Lam, justice secretary Rimsky Yuen and Chinese President Xi Jinping whose pictures were placed on placards saying “authoritarian clown”.

Others carried a black banner mimicking the Chinese national flag with five yellow stars drawn on it. Around 5,000 people took part in the rally according to estimates by AFP reporters at the scene.

University student Vince Ho, 21, said the authorities' hardline approach towards activists was likely to spur others into action.

But Ann Ngan, 64, was not optimistic about the outlook for Hong Kong's democracy movement.

“How can a child fight with a giant? I'm coming out to express my resentments. I don't think (marches) are very useful but I still have to come out,” Ngan said.

Protesters were also seen carrying a banner saying “Hong Kong Independence” alongside a separatist Catalan “Estelada” flag. The region of Catalonia is pushing for secession from Spain.

Protesters jailed

Tens of thousands joined the Umbrella Movement in September 2014, but campaigners were left frustrated they could not force concessions from China.

The city has since seen a new wave of young activists calling for self-determination or full independence for Hong Kong.

The face of the 2014 movement, Joshua Wong, was sent to prison in August for his role in the initial demonstration along with fellow protest leaders Nathan Law, a lawmaker, and Alex Chow, after the Court of Appeal overturned previous non-custodial terms for the trio.

Their jailing was a blow to the pro-democracy campaign and seen as more evidence that Beijing was tightening its grip on semi-autonomous Hong Kong.

It has also drawn criticism from international rights groups and politicians and prompted accusations that the independence of Hong Kong's courts has been compromised under pressure from Beijing.

Authorities vehemently denied there was political interference in the court decision.

Protesters on Sunday called for justice secretary Rimsky Yuen to resign.

City leader Lam on Sunday morning had called for unity in her first national day speech since she became chief executive in July.