New Zealand universities are asking their exchange students in Hong Kong to return home amid the ongoing violent protests there.

Auckland and Waikato universities say they have 13 students in the territory.

Hong Kong university students have barricaded campuses and stockpiled makeshift weapons as protests continue to escalate, with two deaths so far.

A University of Auckland spokesperson said it has 11 students in Hong Kong and 10 have said they will leave.

The arriving students' will not be affected fiscally or academically as a result of leaving their exchange programmes in the former British colony, the spokesperson said.

Most of Auckland's students are expected to be back on New Zealand soil this weekend.

Waikato's international director, Sharon Calvert, said two of its students are in the territory and have been given strong advice to return.

On Thursday, the University of Hong Kong posted a notice on its website which effectively closed the campus down. It said it would only maintain "essential and emergency" services on campus.

It said: "In view of the uncertain and unsafe traffic conditions for travelling to HKU, we are suspending classes on the main campus for the rest of the semester and making teaching and learning accessible online.

"This will enable students to stay away from campus, and even leave Hong Kong should they wish, while at the same time completing their courses and assessments."

Ms Calvert said the decision to ask Waikato students to return home was made after the escalation of violence this week, although the university cannot force the students to return due to their age.

"We're conscious of our responsibilities as a university when we have students overseas and we are constantly monitoring situations and we are always in close consultation with our students when things are awry."

Mrs Calvert said the decision was made earlier this week as violence escalated.

She said Waikato may have to re-evaluate students travelling to Hong Kong next semester given the situation.

"We will be continuing to discuss with our partner [university in Hong Kong] and monitoring the situation closely."

The Foreign Affairs and Trade Ministry's Safetravel website warns New Zealanders in Hong Kong to exercise increased caution and "to avoid all protests and demonstrations, as even those intended to be peaceful have the potential to turn violent with little or no warning".

At least five campuses blockaded by students and protesters

The president of Hong Kong's Chinese University, which anti-government protesters have turned into a fortress stockpiled with petrol bombs and bows and arrows, threatened on Friday to call in "assistance" unless all non-students leave.

But many had already left after some of them allowed the partial reopening of a key highway next to the campus, blocked by protest debris, suggesting differences of opinion in a movement which has been largely leaderless in more than five months of sometimes violent unrest.

That decision was debated by students later in the day and they started blocking the road again, leading to further chaos during the Friday night rush-hour.

The hilly campus was the scene of clashes this week, with pro-democracy protesters hurling petrol bombs at police and on to the Tolo highway linking the largely rural New Territories with the Kowloon peninsula to the south and Hong Kong island beyond.

The highway partially reopened on Friday, but the Cross-Harbour Tunnel, outside the barricaded Polytechnic University where protesters have practised firing bows and arrows and throwing petrol bombs in a half-empty swimming pool, remained closed.

"I am disappointed about the decision to reopen the Tolo highway and it's not our consensus," one student who gave his name as Cheung, 18, told Reuters.

"I was asleep when they had closed-door meetings. I was worried and scared after I realised what had happened and most protesters had left. I was worried the police might storm in again because so few people are left. Some protesters from the outside have gone too far."

Students and protesters have barricaded at least five campuses after four days of some of the worst violence in the former British colony for decades.

Remaining students hunkered down on the bridge over the Tolo highway, black pirate-style freedom flags flying from lookout posts. A few on ladders with binoculars kept watch over the road.

University president urges outsiders to leave

Chinese University president Rocky Tuan said in an open letter that all outsiders must leave.

"Universities are places to study, not to resolve political disputes, or even a battlefield to create weapons and use force," he said.

"If the university cannot continue to fulfil its basic mission and tasks, we must seek the assistance of relevant government departments to lift the current crisis."

There were only about 200 protesters there on Friday compared to at least 1000 two days ago.

One non-student protester, Fung, also 18, said he was also shocked at the decision to allow the highway to reopen.

"We couldn't stop it. I agree there's a need for better communication and coordination between CUHK protesters and non-CUHK protesters."

Demonstrators paralysed parts of Hong Kong for a fifth day on Friday, forcing schools to close and disrupting transport.

The week has also seen a marked intensification of the violence.

A 70-year-old street cleaner, who had been hit in the head by one of several bricks police said had been thrown by "masked rioters", died on Thursday. On Monday, police blamed a "rioter" for dousing a man in petrol and setting him on fire. The victim is in critical condition.

On the same day, police shot a protester in the abdomen. He is in stable condition.

"We can no longer can say Hong Kong is a safe city," Chief Secretary for Administration Matthew Cheung told a briefing.

Police said the Chinese University had been "taken hostage".

"Rioters' violence has infiltrated into almost every corner of society and now turned the Chinese University of Hong Kong into a powder keg," police spokesman Chief Superintendent Tse Chun-chung told a briefing.

Protesters are angry at perceived Chinese meddling in the city since it returned to Beijing rule in 1997 under a "one country, two systems" formula guaranteeing its colonial-era freedoms.

China denies interfering and has blamed Western countries for stirring up trouble.

Police have kept their distance from the campuses for more than two days, saying both sides should cool off, but many observers are afraid of what will happen if and when they move in.