Riot police surround Hong Kong’s Polytechnic University as some protesters hold out An unknown number of protesters remain barricaded inside Polytechnic University.

LONDON -- An unknown number of protesters remain holed up inside Hong Kong’s Polytechnic University as riot police encircle the campus, which has been the site of violent clashes in recent days.

Thousands of black-clad demonstrators -- thought to be mostly students -- once occupied the third largest university in Hong Kong, transforming the Kowloon peninsula campus into a heavily fortified base. They had blocked entrances and armed themselves with bricks, homemade bombs, axes and bows and arrows.

The protesters, who fear mainland China is increasing its control over Hong Kong, battled with police over the weekend, facing tear gas, rubber bullets and even live ammunition. It was one of the largest and most sustained sieges in the anti-government protest movement that has convulsed the semi-autonomous Chinese territory for months.

Since then, about a thousand demonstrators have left the campus, either surrendering to authorities or being caught while trying to escape.

There is now only one way into Polytechnic University and one way out. Visitors and protesters are screened at a basic security checkpoint before being allowed to enter.

The protest movement began in early June when hundreds of thousands of mostly young people marched against a proposed extradition bill that would have allowed suspected criminals in Hong Kong to be sent to mainland China for trial.

Hong Kong's embattled leader, Carrie Lam, has since withdrawn the bill, but widespread unrest has continued as demonstrators broaden their demands to include a call for direct elections for the city's leaders, amnesty for protesters and an independent investigation into alleged police brutality.

Hong Kong's largest, pro-establishment political party has urged voters to"kick out the black force” in district council elections set for Sunday.

Meanwhile, China has ordered the United States not to interfere with the ongoing protests, after Congress unanimously passed a bill intended to back the protesters in Hong Kong and reexamine support for the city's government.

Chinese foreign ministry spokesman Geng Shuang called on U.S. President Donald Trump to veto the proposed legislation, warning it could undermine trade talks between the two nations.

"We urge the U.S. to grasp the situation, stop its wrongdoing before it's too late and immediately take measures to prevent this act from becoming law," Geng said in prepared remarks at a press briefing Thursday. "The U.S. should immediately stop interfering in Hong Kong affairs and China's other internal affairs, or the negative consequences will boomerang on itself. China will have to take strong countermeasures to defend our national sovereignty, security and development interests if the U.S. insists on making the wrong decisions."