“As some of the cases had travel history a few days before … with the incoming number of residents returning to Hong Kong, we may see a high number of cases for at least two weeks or more,” she said.

Experts ramped up their warnings after weeks of reminding the city against complacency as Hong Kong appeared to make significant progress bringing infections under control.

“This is the worst time to relax because we’re at the highest risk since this began,” said Professor Gabriel Leung, dean of the University of Hong Kong’s (HKU) medical faculty and a member of Chief Executive Carrie Lam Cheng Yuet-ngor’s expert panel on the pandemic.

“It is absolutely critical especially as we have this influx of Hong Kong returnees starting from a few days ago and continuing for the next few days. This is the highest risk and we must be extra vigilant.”

He warned that even a few patients spreading the disease could lead to large numbers of local clusters, as South Korea experienced, with more than 8,600 infections and close to 100 deaths.

Top microbiologist Ho Pak-leung, also of HKU, advised the city to brace for hundreds of imported cases of Covid-19 in the next two to three weeks, likening the crisis ahead to a war.

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Ho suggested isolating arriving travellers at hotels to avoid the risk of infection through sharing bathroom facilities as well.

“When one patient passes the virus to two people in the community to form an unknown number of hidden transmission chains, then it’s very likely that we will have 400 to 600 cases in the next two weeks,” he said, adding if that happened, then “Hong Kong is at war”.

With anyone coming into Hong Kong from overseas required to undergo a mandatory two weeks of self-isolation, concerns were also raised about enforcement as a third of quarantined new arrivals could not activate their tracking wristbands.

It was hours before authorities, who admitted that only some 2,000 of about 6,000 digital bracelets that had been handed out could be activated, were able to fix the problem.

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Many complained that after following instructions to download an app, they had not received the passwords by SMS required to activate the devices that would ensure they stayed at their designated addresses.

More than 13,100 travellers – including 11,924 Hongkongers – arrived in the city on Thursday, after the new entry restrictions were put in place. That figure was 36 per cent lower than Wednesday’s count of 20,583, as planeloads rushed in to beat a midnight deadline.

French consul general Alexandre Giorgini issued a message to fellow nationals in Hong Kong, urging them to put on masks, as he described the spike of 48 cases as a turning point.

“Our health, our families, our image in the eyes of the Hong Kong people are at stake,” he said. “It is our duty to take our full part in this fight. The means are simple: let’s wear a mask; for those who have recently travelled but also their close ones, scrupulously comply with the quarantine rules.”

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The CHP said 36 of the new cases, involving patients aged between four and 69, had a recent travel history. Many had been to European countries such as Britain, Switzerland, Austria, Belgium, Portugal and Turkey, while the rest had flown in from the US, Canada, Dubai, Thailand, Singapore, Indonesia and the Philippines.

One of the local cases was a taxi driver who had picked up passengers from the airport at least once a day recently.

Asked whether the government should ban non-residents from entering the city, Chuang of the CHP said all the new patients were residents except for an Australian who had been to the US and Portugal – he was a transit passenger at the airport when he was sent to hospital feeling ill.

Two new patients had visited the Hollywood Road branch of the Pure group, a Hong Kong yoga and fitness chain which had two earlier cases linked to its Lan Kwai Fong centre. One had also visited Pure’s Lan Kwai Fong centre during her incubation period.

“We believe the transmission was not because of the environment, but because people gather together,” Chuang said of the cluster of infections linked to the city’s most popular wining and dining nightspot.

It was possible that people gathering at Lan Kwai Fong without wearing masks were spreading the coronavirus, she said, while it could also have been transmitted in the yoga centre’s changing rooms.

The Pure group announced on Friday that all of its branches would be closed for two weeks.

Two other patients had joined a dinner at Lan Kwai Fong on March 13 with another infected person who then attended a wedding cocktail party in Discovery Bay a day later – two more people at the gathering contracted the coronavirus.

Television Broadcasts Limited, the city’s dominant free-TV station, said it was cleaning its premises after a journalist came into contact with an infected patient.

It is understood that the patient is a Young Post freelancer, and that case prompted the closure of the Times Square headquarters of the Post.

Another new case involved a member of Pure Fitness gym in Central, where two other members were infected.

A Standard Chartered bank employee at the Chung On Street branch in Tsuen Wan was among the new cases – she had attended a wedding banquet on March 14 that three other confirmed patients had also been to. The bank said the branch was closed until further notice, and all staff there would go into 14-day self-quarantine.

Leung of HKU suggested that businesses such as restaurants slash opening hours and limit the number of patrons, while maintaining good hygiene. He noted that some countries had legally limited the opening times of places that might draw crowds, such as bars.

“The government should consider if these stringent measures apply to Hong Kong,” Leung said, but these should be a “last resort”.

People delivering supplies to the elderly should avoid face-to-face contact altogether, “however hard it may be”, he added. Though people might want to visit the elderly to lift their spirits, doing so could also risk infecting them, he warned.

“What I do not want to see is love turning into harm,” he said.

Separately, health minister Sophia Chan Siu-chee told a panel meeting at the legislature that the government would consider barring entry to non-Hong Kong residents if it was deemed necessary, while calling on those under home quarantine to practise self-discipline.

By 9pm on Friday, some 50 arrivals at the airport were earlier screened and sent for testing at two new temporary facilities the Hospital Authority had opened to handle the influx of people returning from overseas.

The facility at AsiaWorld-Expo tested 46, while the North Lantau Hospital handled four.

Four have been sent to hospital for treatment after preliminarily testing positive for the virus.

Additional reporting by Alvin Lum and Victor Ting