The city’s No 2 official, Matthew Cheung Kin-chung, said the toll booths at the tunnel that connects Hung Hom to Causeway Bay had been “completely destroyed” and would have to be “almost entirely rebuilt”. It would also take time to repair the electronic equipment, he added.

“But there is no reason to not resume service just because of the problems with the booths,” he said.

The government could stop charging fees temporarily once it was safe to reopen the tunnel, he said. Tolls range from HK$8 to HK$30 (US$1 to US$3.84).

Only a small number of protesters responded to online calls to “paralyse” the city by disrupting road traffic and rail services on Thursday morning.

In the Central business district, about 300 people gathered during lunch time at the atrium of the International Financial Centre mall to show support for those holed up inside PolyU.

“I came out to help keep the spirit of the movement alive,” said Bella, 26, who works in the financial industry. “I did not think it was a smart move to get inside PolyU in the first place, but now that we’ve got to this stage we’ll provide all the support we can.”

The turnout at Central was much smaller than at its peak last week, when thousands gathered on the streets for successive days.

In the evening, several hundreds staged a sit-in inside Yuen Long’s Yoho mall to mark four months since a mob of about 100 people stormed into the nearby MTR station and indiscriminately attacked people with sticks and iron rods. Protesters were angry that police took a long time to arrive, and that the attackers had fled by the time officers were on the scene.

Among those present then was Martin Chan, a civil servant who has attended a protest on the 21st of every month since the attack.

“I still remember how police left Yuen Long residents who were under attack and seeking help. They only reappeared 39 minutes later,” he said, adding he did not fear being arrested for taking part in an unauthorised assembly.

Eve Wong, 14, was also at the sit-in but said he told his family he was out with his friends.

“I just want to come here to support the movement and do my share,” he said.

At about 8.45pm, a crowd of more than 100 protesters started to march from the mall onto the main roads of Yuen Long but riot police showed up soon after and urged them to keep to the pavements. Some responded by hurling abuse at the officers but nothing more came of the stand-off and protesters dispersed before 10pm.

On social media, there were calls for protesters not to block roads on Sunday and the days before for fear they could be arrested and thus unable to vote. Officials have also appealed to protesters not to paralyse public transport to allow the election to take place.

The elections are widely seen as a de facto referendum on the government and the results will be a gauge on the level of support enjoyed by the anti-government movement that has rocked the city for nearly six months, sparked by the now-withdrawn extradition bill.

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At PolyU, many diehard protesters were hiding from public view to rest. Some chose to sleep on the higher floors of buildings.

A black-clad protester, who said he was under 18, told the South China Morning Post that he missed his home badly.

Under a deal reached by education figures and police, anyone younger than 18 can go home after officers record their personal information. But the police can still pursue criminal liabilities in the future.

“I can’t take any chance of having my name taken,” the protester said, adding that he did not want to have a criminal record.

Another protester, in his early 20s, was still looking for a way to avoid capture on his seventh day at PolyU. He said he neither understood nor cared about politics before the protest movement erupted in June. But he soon realised that “nothing much can be achieved through peaceful means”.

Another protester, 25, had no regrets about entering PolyU hours before police laid siege to the campus on Sunday evening.

“I would have regretted not coming,” said the man, who works in the engineering industry.

He had been on the protest front lines to help the injured and stressed that he had never attacked police. All he wanted was to do was to protect young people, said the man, who identifies as a Christian.

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An HKU social work professor who had been in the university since Wednesday said a 12-year-old girl had left for hospital at 5am on Thursday. She had not suffered any major injuries but she was quite weak from exhaustion.

City University’s President Way Kuo arrived at PolyU on Thursday afternoon to look for two of his students but left after failing to find them.

PolyU Vice-President Alex Wai Ping-kong said he did not know how many people remained inside the campus.

“We know that some of those who stayed had diarrhoea and stomach aches and were taken away by ambulances,” Wai said.

He did some cleaning work at a canteen on the campus and said the poor hygiene meant no food could be served there. He disclosed that almost all laboratories had been broken into and that “dangerous chemicals” were missing.

A check by the Post showed at least 20 laboratories had been ransacked. A least three stored flammable or explosive chemicals, while others stored toxic or corrosive substances. It remained unclear whether the materials had been used to make petrol bombs.

Separately, Chinese University confirmed that its vice-chancellor, Rocky Tuan Sung-chi, had been admitted to hospital.

“Professor Tuan is ill and he is taking a rest and is recovering,” a university spokesman said.

The Post learned that Tuan has been at the Prince of Wales Hospital since last weekend. He was at the campus on November 12 during violent clashes between protesters and police to mediate between them. He and other leaders from the university were seen running from tear gas fired by police.

Education minister Kevin Yeung Yun-hung said he supported schools that punished students who blocked roads and disrupted MTR services.

Meanwhile, a group of social workers, including Jacky Chen Hung-sau from the Social Workers’ General Union, was planning a three-day strike next month. They urged people from other sectors to join them.

Additional reporting by Cannix Yau and Ng Kang-chung