Thousands of people have rallied in Hong Kong to protest against the jailing of three prominent pro-democracy activists.

In sweltering heat, the demonstrators marched from the district of Wan Chai to the Court of Final Appeal on Sunday in support of Joshua Wong, 20, Nathan Law, 24 and Alex Chow, 27, who were sentenced to between six and eight months in prison on Thursday.

The three were found guilty of unlawful assembly last year for their role in the "Umbrella Movement" civil disobedience movement in 2014, demanding full democracy.

They were handed the jail terms by an appeals court after the Hong Kong's Department of Justice applied for a review, seeking imprisonment.

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Hong Kong, a former British colony, returned to China in 1997 under a "one country, two systems" agreement that ensured its freedoms, including a separate legal system.

But Beijing has ultimate control, and some people in Hong Kong are concerned it is increasingly interfering to head off dissent.

The financial hub was rocked by nearly three months of mostly peaceful street occupations in late 2014, demanding Beijing grant the city full democracy.

The "Umbrella Movement", which drew hundreds of thousands of protesters at its peak, was triggered after Wong and his colleagues stormed into a courtyard fronting the city's government headquarters.

'Give back hope to my children'

Former student leader Lester Shum, who helped organise Sunday's rally, said the number of protesters was the highest since the 2014 protests.

"This shows that the Hong Kong government, the Chinese Communist regime and the Department of Justice's conspiracy to deter Hong Kong people from continuing to participate in politics and to protest using harsh laws and punishments has completely failed," Shum told Reuters news agency.

The protesters held signs which read: "Give back hope to my children" and "One prisoner of conscience is one too many".

William Cheung, an engineer in his 40s, told AFP news agency that the ruling is "the beginning of white terror" in Hong Kong.

"These young people are our hope for the future. We shouldn't treat them like this," Jackson Wai, a retired teacher in his 70s, told AFP.

Protesters brandished a large banner saying: "It's not a crime to fight against totalitarianism."

They shouted: "Release all political prisoners. Civil disobedience. We have no fear. We have no regrets."

Al Jazeera's Divya Gopalan, reporting from the rally, said: "What's interesting is that besides the pro-democracy activists and students usually seen at such protests, we're also seeing a lot of the older generation and parents."

"Hong Kong is changing," an elderly woman at the protest told Al Jazeera. "The freedom of democracy is all gone. We don't trust the government."

'Standing together'

Ray Wong, 24, leader of the pro-independence group Hong Kong Indigenous, said the issue is uniting government opponents.

"Since the Umbrella movement, the radical and milder forces walked their own path," he said, referring to the 2014 democracy movement. "We're now standing together. It is a good start."

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Just before the sentencing, a defiant Wong told around 100 supporters who thronged into the High Court lobby on Thursday that he had no regrets and urged them to keep fighting for full democracy.

The jail terms for Wong, Law and Chow disqualify them from running for the legislature for the next five years.