HONG KONG -- More than 20 protesters inside a Hong Kong university campus surrendered to police on Thursday as the city's largest pro-Beijing political party urged voters to “kick out the black force” in upcoming elections seen as a key gauge of public support for anti-government demonstrations.

At least 23 people left Hong Kong Polytechnic University, which has been ringed by riot police for days, as the campus siege edged closer to an end.

Ten protesters walked out together and were escorted to a police post outside the campus, while three were carried out on stretchers and four taken in wheelchairs. Five other students, believed to be minors, came out with their parents and were allowed to leave after police took their details.

It is unclear how many protesters are left behind. They are the holdouts from a much larger group that occupied the campus after battling police over the weekend. Some 1,000 protesters have either surrendered or been stopped while trying to flee.

The city's largest political party slammed the flareup in violence in the past week and urged some 4.1 million voters to use the ballot box this Sunday to reject the “black force” that had thrown the semi-autonomous Chinese territory into unprecedented turmoil since June.

“The black force say they want to fight for freedom but now people cannot even express their views freely. We have even been stripped of our right to go to school and work,” said Starry Lee, who heads the Democratic Alliance for the Betterment and Progress of Hong Kong.

Lee and some candidates kicked black footballs at a campaign event in a symbolic banishing of the black-clad protesters. “If everyone comes out to vote, Hong Kong can be restored and violence can be stopped,” she said.

The party is contesting 181 of the 452 district council seats, a low-level neighbourhood election held every four years and dominated by the pro-establishment camp. For the first time, all the seats will be contested. Public anger against the government and police could give a victory to the pro-democracy bloc that would bolster the legitimacy of the protest movement.

“We cannot give even a single vote to the pro-establishment camp,” said an angry mother, who with her young son joined hundreds of people at a sit-in protest at a mall against an attack by thugs on protesters four months ago. A live video of the protest was broadcast by Apple Daily newspaper.

Protesters, who believe mainland China is increasing its control over the territory, are demanding fully democratic elections and an independent probe into alleged police brutality against demonstrators. The government rejected the demands and has warned the polls could be delayed if violence persists and transport links are disrupted.

A Hong Kong restaurant owner was deported from Singapore for organizing an illegal gathering last month to discuss the protests, Singapore media reported. Alex Yeung, who founded the Wah Kee restaurant chain and a staunch pro-Beijing supporter, will also be barred from entering Singapore without prior approval.

In a video posted on YouTube from Singapore's Changi Airport, Yeung said he has been warned to refrain from any criminal conduct. He didn't say where he was heading but urged Hong Kong residents to cast their vote on Sunday to “reject violence and support peace.”

More than 5,000 people have been arrested since the protests started in June over a now-abandoned extradition bill that would have allowed criminal suspects to be sent to mainland China for trial. The protests have since swelled into an anti-China movement as many fear a loss of freedoms guaranteed to the former British colony when it returned to Chinese control in 1997.

A 12-year-old became the youngest protester to be convicted Thursday after pleading guilty to spraying graffiti outside a police station and subway exit last month, the South China Morning Post reported. A lawyer for the student reportedly said he was remorseful and acted on impulse. The court will sentence him on Dec. 19.

Pressure ratcheted up on Hong Kong as the U.S. Congress approved legislation late Wednesday to sanction officials who carry out human rights abuses and require an annual review of the favourable trade status that Washington grants Hong Kong. Another bill bans export of tear gas and other non-lethal tools to Hong Kong.

The Chinese foreign ministry's representative in Hong Kong, Xie Feng, summoned U.S. Consul-General Hanscom Smith to oppose the bills, which are expected to be signed into law by President Donald Trump.

Xie told Smith that the bills support “rogue democracy” and a right to riot that would hamper Hong Kong's effort to halt the violence, the official Xinhua News Agency reported. He also warned Beijing would take “resolute countermeasures.”

The U.S. confirmed the two met and said it believes Hong Kong's autonomy and commitment to protecting civil liberties are key to preserving its special trade status under U.S. law.

Associated Press writer Ken Moritsugu and video journalist Katie Tam contributed to this story.