HONG KONG (REUTERS) – Fires blazed on the doorstep of a Hong Kong university into the early hours of Sunday as protesters hurled petrol bombs and police fired volleys of tear gas in some of the most dramatic scenes in more than five months of escalating violence.

Hours earlier, squads of Chinese soldiers dressed in shorts and T-shirts, some carrying red plastic buckets or brooms, emerged from their barracks to help clear debris that has blocked some key roads in the city for days.

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.

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.

Huge fires lit up the night sky at Hong Kong Polytechnic University in Kowloon district as protesters hurled scores of petrol bombs, some by catapult, and police fired round after round of tear gas before pushing the protesters up onto the podium of the red-brick campus.

It had the feel of a fortress, 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.

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.

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 on Thursday by President Xi, in which he denounced the unrest and said “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 Saturday but Hong Kong’s Cross-Harbour Tunnel was still blocked by protesters occupying Polytechnic University.