Hong Kong: Chinese troops have emerged from a Hong Kong barracks to remove protester barricades in a rare appearance on the city's streets that has spooked democracy activists and was slammed as potentially violating the law.

Student protesters holed up in the Polytechnic University, metres from another People's Liberation Army barracks, told The Sydney Morning Herald and The Age they feared the appearance of the military was "the first step".

People’s Liberation Army soldiers, with brooms, arrive to clean up the protest area at Hong Kong Baptist University in Hong Kong, on Saturday. Supplied

Hours later riot police moved against the Polytechnic University campus, where students had been stockpiling petrol bombs and blockading the Hung Hom cross harbour tunnel for days. Students threw petrol bombs metres from the PLA gates as riot police fired tear gas and rubber bullets.

People's Liberation Army troops are rarely seen outside barracks in Hong Kong, and the threat of a Chinese military crackdown weighs heavily over the long-running protests, particularly after Chinese President Xi Jinping declared on Thursday it was Hong Kong's "most pressing task" to restore order.

So the sight of dozens of Chinese military in khaki T-shirts running out of the barracks in Kowloon Tong near Baptist University and working to lift roadblocks at 4pm on Saturday prompted a news alert from the public broadcaster Radio Television Hong Kong.

A PLA officer quoted Xi to reporters: "Stopping violence and riots are the responsibility of every person."

In this image made from video, People’s Liberation Army soldiers dressed in shorts and T-shirts pick up bricks scattered by protesters at Hong Kong Baptist University in Hong Kong, on Saturday. (Television Broadcasts Limited Hong Kong via AP)

Opposition party politicians demanded to know if Lam's government had invoked Article 14 of the Garrison Law, "which stipulates that the PLA must not interfere in local affairs unless it is asked by the Hong Kong government to help with disaster relief or maintaining public order".

The article has never been invoked since the handover to China in 1997. A Hong Kong government spokesman said it had not requested the PLA's help and they had come out voluntarily.

Lawyer and author Antony Dapiran said the government's comment that the PLA was on the street as volunteers was "nonsense". He said unless there was an exemption in the Garrison Law for volunteers, the move was illegal.

"Removing protester barricades is not 'cleaning', it is 'policing'," he wrote on social media.

Saturday's appearance seemed highly staged, with Chinese propaganda outlets including the People's Daily, circulating video of the soldiers "removing bricks and other objects deliberately put on streets by rioters".

Rare weekday fighting in Hong Kong carried into Tuesday night as authorities admitted that the city is on the verge of collapse.

The Hong Kong government information office later distributed a press release from the PLA to media, which said officers and soldiers from the garrison had joined a clean-up near the gate of the military camp.

Pro-Beijing cabinet minister and former security minister Regina Ip told local media: "They are not carrying out military duties. Their help was greatly appreciated."

The students at the Polytechnic University held an urgent meeting on Saturday to decide whether to continue their occupation of the campus or leave.

Three students in black masks on a boundary patrol outside the campus at 8.30pm said they were concerned there was a PLA barracks just metres across the road, which was littered with the protester's brick barricades, and had been blocked for three days.

Members of the PLA stand guard at an entrance to their barracks in the Kowloon Tong area of Hong Kong. Bloomberg

At a security checkpoint manned by protesters inside the campus to prevent infiltration by undercover police, two Polytechnic students said they felt the PLA appearance at Kowloon Tong was "the first step".

Jace, 21, said: "It was absolutely a signal. They are trying to warn us not to do any further violent acts."

Protesters shoot with a catapult during a clash with police at the Hong Kong Poytechnic University on Saturday night. Getty Images

He said there were no leaders inside the protest movement so there was no consensus on whether to evacuate the campus. But there was a wide sense of unease that riot police had not been near the university in three days.

"Now the Chinese military have come out and identified themselves, later on they might use weapons to fight us," said another student.

"We are not afraid they will come inside to attack us, because we have enough manpower to stay here and defend the campus," he said.

Hon, a church volunteer from Mongkok in an orange vest said: "We have seen the video online. Obviously in the city right now there are some people who don't agree with the protesters."

Hon had stayed at the campus until 3am for the past three nights, but didn't believe the police would attack on Saturday because the roads were so heavily barricaded. But shortly after train services were cut at 10pm, riot police moved in, firing tear gas and rubber bullets on the street that separates the university from the PLA barracks.

On the opposite side of the campus, bombed out toll booths to the harbour crossing had been guarded by protesters.

The footbridge linking the university to the train station was fortified with petrol bombs and arrows, and all access roads lined with nails and brick barricades.

Only an ambulance station was spared the fortifications.Polytechnic's president Teng Jin-Guang had sent an email to all students on Friday asking them to leave the campus, which he said had been overtaken by activists who had "severely and extensively vandalised" facilities.

Several hundred people, who mostly appeared to be Polytechnic students, were inside the campus on Saturday night as police began to fire tear gas.

Days earlier thousands of protesters had occupied the brick building, erecting new bricks walls as barricades and making petrol bombs, in reaction to a fierce clash between riot police and protesters at the Chinese University.

Ahead of the police arriving at Hung Hom, a middle-aged couple watched the black clad protesters guarding the bombed toll booths. The woman said she was born in Hong Kong and started to cry: "This is not Hong Kong, this is terrible."

Police returned to Polytechnic on Sunday morning and a battle with students continued.