HONG KONG (REUTERS) - Fresh violence erupted around a besieged Hong Kong university campus on Sunday (Nov 17) morning, as protesters braced for a possible final police push to clear them after fiery clashes overnight.

The city's Education Bureau announced that classes at all schools will continue to be suspended on Monday for the safety of students. All kindergartens, primary and secondary schools as well as special schools have been closed since last Thursday.

In a statement on Sunday, the bureau said that schools should prepare for classes to resume, but students should stay at home and not take part in any unlawful activities. "Although the roads and public transport services in the territory have gradually resumed, there are still uncertain factors currently," the bureau said.

The announcement came after clashes in the morning at the Hong Kong Polytechnic University in Kowloon district. Protesters hurled petrol bombs, some by catapult, and police fired volleys of tear gas to force them up onto the podium of the red-brick campus.

After skirmishes overnight, protesters slept for a few hours on lawns and in the university library. Police fired fresh rounds of tear gas shortly after 10am. Activists hurled petrol bombs in return, some igniting trees outside the campus.

Still earlier, squads of Chinese soldiers dressed in shorts and T-shirts, some carrying red plastic buckets or brooms, emerged from their barracks in a rare public appearance to help residents clear debris blocking key roads.

Parts of the Hong Kong Polytechnic University campus looked more like a fortress on Sunday morning, with barricades and black-clad protesters manning the ramparts with improvised weapons like bricks, crates of fire bombs, and bows and arrows at the ready.

"We don't want to attack the police, we just want to safeguard our campus... .and we want to safeguard Hong Kong," said Chan, 20, a year-three student at the university who did not want to provide her full name.

The campus is the last of five universities to be occupied, with activists using it as a base to continue to block the city's central cross-harbour road tunnel.

The presence of China's People's Liberation Army (PLA) soldiers on the streets, even to help clean up, could stoke further controversy over Hong Kong's autonomous status at a time many fear Beijing is tightening its grip on the city.

Pro-democracy lawmakers condemned the PLA's actions in a joint statement, warning that under the city's Garrison Law, the military must not interfere in local affairs unless it was asked by the government to help with disaster relief or public order, government-funded broadcaster RTHK reported.

Hong Kong did not request assistance from the PLA and the military initiated the operation as a "voluntary community activity", a spokesman for the city's government said.

The Asian financial hub has been rocked by months of demonstrations, with many people angry at perceived Communist Party meddling in the former British colony, which was guaranteed its freedoms when it returned to Chinese rule in 1997.

Beijing denies interfering and has blamed the unrest on foreign influences.

Clashes between protesters and police have become increasingly violent in the Chinese-ruled city, which is grappling with its biggest political crisis in decades.

The demonstrations pose the gravest popular challenge to Chinese President Xi Jinping since he came to power in 2012.

Mr Xi has said he is confident the Hong Kong government can resolve the crisis, and until Saturday, Chinese troops in the city had remained inside their base during the protests.

Chinese state media repeatedly broadcast comments made last Thursday by President Xi, in which he denounced the unrest and said that "stopping violence and controlling chaos while restoring order is currently Hong Kong's most urgent task".

Efforts on Saturday to clear roads that have been blocked for days, causing massive disruption, followed some of the worst violence seen this year after a police operation against protesters at the Chinese University of Hong Kong on Tuesday.

The authorities have since largely stayed away from at least five university campuses that had been barricaded by thousands of students and activists stockpiling makeshift weapons.

Many protesters appeared to have left the campuses by late on Saturday but Hong Kong's Cross-Harbour Tunnel was still blocked by protesters occupying Polytechnic University.

Earlier, hundreds of pro-China demonstrators gathered by the city's legislature and police headquarters, waving Chinese and Hong Kong flags.

Some held up posters reading "Police we stand with you", while others chanted "Support the police". Pro-China protests have so far attracted much smaller numbers than those angry at Beijing.

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By late afternoon on Saturday, PLA soldiers had left the streets outside Baptist University beside their barracks in Kowloon Tong.

Chinese troops have appeared on streets only once since the 1997 handover, to help clear up after a typhoon in 2018. It was not clear how many were involved on Saturday.

The PLA garrison in Hong Kong said that when residents began cleaning, some troops "helped clear the road in front of the garrison gate".

Demosisto, a pro-democracy organisation, said Saturday's clean-up operation could set a "grave precedent" if the city's government invites the military to deal with internal problems.

In August, Beijing moved thousands of troops across the border into Hong Kong in what state news agency Xinhua described as a routine rotation.

Standing beside a black flag with the slogan "Liberate Hong Kong, Revolution of our Times", Mr James Wong, 23, was among protesters manning a bridge at Baptist University.

"We didn't want to confront the people and the PLA troops directly," he told Reuters. "We are not directly against the PLA, but rather the government. But the PLA should not leave their base because this is Hong Kong territory."

Hundreds of residents moved in to help clear barricaded roads near several universities.

Earlier clashes on Saturday saw at least one petrol bomb thrown before anti-government protesters at the campuses retreated. No soldiers appeared to have been involved in the confrontations.

"We just want our lives to continue," said one resident who was helping clear streets near Hong Kong University.

"There are many elderly who need to go to the hospital and children who need to go to school. I am very sad to see what is happening in my community."