Three young Hong Kong protesters were jailed for three years Friday on "riot" charges for their part in anti-China protests last year as tensions rise ahead of a vote for the city's next leader.

The sentencing came just over a week before a vote for Hong Kong's new chief executive, which pro-democracy campaigners dismiss as a rigged election weighted towards Beijing.

Activists have said they will come out in protest on March 26, the day of the leadership selection, which is made by a committee of 1,194 mainly pro-establishment members of special interest groups.

The three activists jailed on Friday had taken part in the "fishball riots" of February 2016, which tapped in to fears Beijing is tightening its grip on semi-autonomous Hong Kong.

None of the three jailed Friday are well-known campaigners.

The riot's alleged ringleaders from "localist" campaign group Hong Kong Indigenous, which advocates more autonomy for the city, face trial next year.

"Anyone participating in such riots needs to understand there is a cost," said district court judge Sham Siu-man Friday.

The defence had argued the three protesters -- two of whom were students and one reported to be a cook -- had been expressing their disapproval of the Hong Kong government, which activists say is a puppet of Beijing.

But Sham said: "Violence is violence," as he delivered the sentence to the grim-faced accused, Hui Ka-ki, 23, Mak Tsz-hei, 20 and Sit Tat-wing, 33, who were then led away.

Sham said all three defendants had hurled glass bottles, with Mak throwing bamboo polls at officers.

The clashes erupted after official attempts to remove illegal hawkers from the busy commercial neighbourhood of Mong Kok during Lunar New Year celebrations.

The battles were dubbed the "fishball revolution" after a favourite Hong Kong street snack sold by the hawkers, and reflected underlying tensions over the erosion of the city's traditions.

Police fired warning shots in the air, while demonstrators hurled bricks levered up from pavements, charged police lines with homemade shields and set rubbish on fire.

About 100 people were injured, including police officers, journalists and protesters, and 65 were arrested in the rare outbreak of violence.

Mong Kok, on the city's Kowloon peninsula, was the scene of some of the worst clashes during largely peaceful mass pro-democracy "Umbrella Movement" rallies in 2014, which failed to win political reform.

Friday's sentencing was the harshest handed out to those who took part in the fishball demonstrations, and the first convictions for rioting.

Three other participants were last year sentenced to between three and nine months on lesser charges.

Activist Ray Wong from Hong Kong Indigenous, who is also facing a riot charge, said protesters were willing to risk jail.

"Every young Hongkonger loves their home, that's why they come out to pay the price," he told reporters outside the court.

Police welcomed the verdict, saying it gave a message to the public that violence would not be tolerated.

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