A potentially deadly standoff between Hong Kong police and dozens of desperate protesters barricaded into a university campus moved into a third day Tuesday, as China issued fresh warnings its patience with nearly six months of unrest was running out.

The siege at Hong Kong Polytechnic University (PolyU) is the most intense and prolonged stand-off of the crisis, which has seen millions take to the streets since June to voice anger at China eroding the territory's unique freedoms.

A new phase that began last week has led to chaos throughout Hong Kong, with schools closed, train lines disrupted and major roads blocked by barricades.

The move by more hardcore members of the protest movement to take over PolyU on the weekend was also a new tactic for them. Previously they had focused on lightning strike protests and acts of vandalism.

With the crisis deepening, China's ambassador to Britain upped the ante on Monday.

"The Hong Kong government is trying very hard to put the situation under control," Liu Xiaoming said.

Hong Kong protests continue
Protesters react as police fire tear gas while they attempt to march towards Hong Kong Polytechnic University. Photo: AFP / DALE DE LA REY

"But if the situation becomes uncontrollable, the central government would certainly not sit on our hands and watch. We have enough resolution and power to end the unrest."

In another ominous signal, China insisted Tuesday it had sole authority to rule on constitutional matters in Hong Kong.

The warning came as it condemned a decision by the city's high court on Monday to overturn a ban on face masks worn by pro-democracy protesters.

Only China's parliament has the right to rule on Hong Kong's Basic Law -- the city's mini-constitution, Zang Tiewei, a spokesman for the body, said in comments carried by state-run media.

"No other institution has the right to make judgements or decisions," Zang said, according to a state media report posted on the parliament's website.

Hong Kong protests continue
A protester walks on stairs at the barricaded main entrance of Hong Kong Polytechnic University. Photo: AFP / Ye Aung Thu

Zang's comments will deepen concerns that Beijing is chipping away at the autonomy of the financial hub -- the fundamental fear driving the popular movement.

A brief appearance by Chinese soldiers on Hong Kong's streets over the weekend -- supposedly to clean up debris -- had also fuelled concerns that Beijing could intervene militarily to end the crisis.

Hong Kong protests continue
Graphic charting the number of protest-related arrests in Hong Kong since June 2019. Photo: AFP / John SAEKI

The siege at PolyU has seen hundreds of mainly young protesters occupy the city centre campus, repelling police surges with a barrage of Molotov cocktails, arrows and bricks.

AFP reporters at the campus said between 100 and 200 people remained on Tuesday, down from several times that figure at the weekend.

Some tried to escape through manhole covers to evade a tightening dragnet that has seen hundreds of people arrested in the last few days.

In her first public comments on the PolyU crisis, Hong Kong chief executive Carrie Lam on Tuesday morning urged the remaining protesters to surrender.

Late Monday, dozens slithered down ropes from a footbridge to a road below, where they were whisked away on motorbikes.

In an apparently co-ordinated effort, tens of thousands of Hong Kongers streamed towards the PolyU campus, as clashes simultaneously raged with police nearby in Kowloon.

Footage showed armoured police beating fallen protesters with batons as they lay on the ground. One officer was filmed stamping on the head of a man who was already subdued.

Alleged police brutality is one of the central complaints of the protest movement, but senior officers say their officers are acting in accordance with the law.

"I totally disagree that our officers are out of control and have used excessive force. We use force when there is violence," incoming Hong Kong police chief Chris Tang told the South China Morning Post.

Inside the campus, exhausted and scared protesters sounded notes of defiant even as they rationed their remaining food and bottled water.

"I will run but never surrender. I don't want to get arrested," said a protester who gave his name only as 'W'.

"We are like rats in a trap."

Protests started in June as a peaceful condemnation of a now-shelved China extradition bill.

They morphed into a confrontational action to defend the city's unique freedoms, which were meant to be enshrined in the Basic Law when Hong Kong was handed over by the British in 1997.

The unrest has tipped the international financial hub into recession, frightened off tourists and pummelled the vital retail sector.

Violence has worsened this month, with two men killed in separate incidents.

Demonstrators last week engineered a "Blossom Everywhere" campaign of blockades and vandalism, which shut down sections of Hong Kong's transport network and closed schools and shopping malls.

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Hong Kong protests continue
Dozens of Hong Kong pro-democracy protesters dramatically escape a police siege at a university campus by shimmying down ropes from a bridge to waiting motorbikes. Photo: AFPTV / Michael ZHANG