Australian university students on exchange in Hong Kong are being urged to come home immediately, as protesters take their pro-democracy fight to campuses across the city.

Key points: Australian university students are being told to leave Hong Kong immediately and come home

Australian university students are being told to leave Hong Kong immediately and come home Police skirmished with militant students at major university campuses around the Chinese-ruled city

Police skirmished with militant students at major university campuses around the Chinese-ruled city The protests are in their fifth month and show no sign of ending

University students in Hong Kong have barricaded campuses and stockpiled makeshift weapons amid escalating violence, including two deaths.

The University of Sydney has started contacting its students in the Chinese-administered city, after Hong Kong University (HKU) and other institutions suspended classes for the last few weeks of the semester.

"Your safety is our utmost concern and based on all the information available to us, and in light that HKU has now suspended/cancelled the semester, the University of Sydney requires you to depart HK immediately and make your way back to Australia," the email sent to students by exchange coordinators on Friday morning said.

Classes at Hong Kong universities for the remaining weeks of the semester have been cancelled. ( ABC News: Steve Wang )

"If you have already left HK, please let me know where you are now.

"If you need assistance and advice on how to depart HK, please let me know and USYD will assist you."

Classes for the remaining weeks of this year's academic semester have been cancelled after protesters at Hong Kong universities, including Hong Kong Polytechnic and the Chinese University of Hong Kong, hurled petrol bombs and bricks and even shot arrows at police in recent days.

Authorities have met the increased violence with tear gas, rubber bullets and water cannons filled with blue dye.

'It could be anyone like me who could be shot next'

Erin Jory only had a couple of weeks left on her exchange, but she has been told to return within a few days. ( ABC News: Steve Wang )

University of Queensland student Erin Jory was due to wrap up an exchange at the University of Hong Kong within weeks, but has now been told by her university that she will need to return by Tuesday.

"It's unfortunate that it's going to end this way, but it's obviously not possible to study on the campus anymore, so it makes sense to leave," the 21-year-old said.

Access to the campus at the University of Hong Kong is blocked by makeshift barricades, with student protesters checking IDs of visitors.

Students built barricades out of chairs and tables, as well as bricks and mortar. ( ABC News: Steve Wang )

But while it looks unruly, Ms Jory said she did not felt unsafe studying there.

"Just even talking to the protesters, they're all kids my age, like me, fighting for what they believe in. It's just really sad to think it could be anyone like me who could be shot next," she said.

A fellow Australian, law student Haytham Chernov, has also been ordered to head home by the University of Sydney.

"It's not how I anticipated on ending my degree. It's somewhat anticlimactic," he said.

Haytham Chernov said the campus looked like it was being prepared for war. ( ABC News: Steve Wang )

Since commencing an exchange semester in August, he has watched as the protests evolved from mass street marches to smaller, more violent confrontations with police.

"There were a few times when I inadvertently ended up in an MTR (Mass Transit Railway) station that was closed down — it was quite scary seeing all the riot police come in and arrest people, and a few times I came across some tear gas," he said.

"But the most emotional thing was coming in for class on Monday morning and seeing the campus transformed, almost preparing for a war."

Despite the upheaval, he said Hong Kong still felt like a safe city with a supportive community.

Protesters at Hong Kong universities have clashed with police in recent days. ( AP: Ng Han Guan )

The warning to return to Australia did not carry with it an exemption for students from completing their studies, as HKU is allowing its courses and exams to be sat online.

"This is good news because it means that you can complete the remainder of your work and still receive the academic credit provided you pass all the units," the email said.

"We expect that you will complete the studies remotely in order to complete the semester's coursework."

Two deaths as violence escalates

Petrol bombs and Molotov cocktails have been used by protesters. ( ABC News: Steve Wang )

A 70-year-old street cleaner, who videos on social media showed had been hit in the head by a brick thrown by "masked rioters", died on Thursday, authorities said.

Earlier this month, a student protester died after falling from a parking lot during demonstrations.

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Police on Tuesday fired tear gas in the heart of the central financial district and at two university campuses to break up pro-democracy protests — a day after shooting a protester at close range, wounding him.

A "rioter" also doused a man with petrol before setting him on fire. The man, who suffered burns to his torso and head remains in a critical condition.

On Thursday, Australia's Foreign Minister Marise Payne said it was "essential" that police responded to the protests "proportionately".

"We reiterate our view that it is crucial for all sides — police and protestors — to exercise restraint and take genuine steps to de-escalate tensions," Senator Payne said in a statement.

Protests in Hong Kong are in their fifth month and show little sign of ending, as pro-democracy advocates continue to demand Hong Kong's chief executive Carrie Lam stand down from the local government and a review of police conduct be launched.

Student actions 'another step closer to terrorism'

Thousands of students remain hunkered down at several universities, surrounded by piles of food, bricks, petrol bombs, catapults and other homemade weapons.

Police said the prestigious Chinese University had "become a manufacturing base for petrol bombs" and the students' actions were "another step closer to terrorism".

Around 4,000 people, aged between 12 and 83, have been arrested since the unrest escalated in June.

Police said they came under arrow fire at the Chinese University of Hong Kong campus. ( AFP: Philip Fong )

The unrest began as a kickback against an attempt by the city's Beijing-backed government to hustle through a bill that would have allowed the extradition of accused criminals to mainland China.

Protesters are angry about what they see as police brutality and meddling by Beijing in the freedoms guaranteed under the "one country, two systems" formula put in place when the territory returned to China from British rule in 1997.

China denies interfering and has blamed Western countries, including Britain and the United States, for stirring up trouble.

But the anger has recently turned towards anyone seen to be affiliated with China.

On Friday, Hong Kong's Secretary for Justice Teresa Cheng, who was in London to promote the Chinese-ruled city as a dispute resolution and deal-making hub, was targeted by a group of protesters who shouted "murderer" and "shameful".

The Hong Kong Government said Ms Cheng suffered "serious bodily harm", but gave no details.

Protesters walk past barricades of bricks on a road near the Hong Kong Polytechnic University in Hong Kong. ( AP: Kin Cheung )

The Chinese embassy in the UK said Ms Cheng was pushed to the ground and sustained a hand injury.

"[Ms Cheng] was besieged and attacked by dozens of anti-China and pro-independence activists," the Chinese embassy said in a statement.

The incident showed that the "violent and lawless perpetrators" were now taking their violence abroad, it said.

China has lodged a formal complaint with Britain and urged British authorities to bring the perpetrators to justice.

On Friday, Video footage of the Chinese People's Liberation Army garrison headquarters near Hong Kong's central business district showed more than a dozen troops conducting what appeared to be anti-riot drills against fake protesters carrying black umbrellas.

A congressional advisory body urged the US Congress on Thursday to enact legislation that would suspend the special economic status Hong Kong enjoys under US law if China deployed security forces to crush the protests.

ABC/Reuters