The outbreak is the most severe one since a cluster involving several bars and musicians that began in March and ultimately involved 103 people.

Dr Chuang Shuk-kwan, head of the communicable disease branch at the Centre for Health Protection, warned that the cluster was expanding rapidly since the first case emerged on Thursday.

“The number of cases has been rising quickly, especially in the dance cluster,” she said. “And some of those do not stay at home as they do not show symptoms and thus they engage in other activities. It is difficult to contain [the spread].”

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She admitted tracking down all the patrons would be difficult and urged the visitors to come forward.

“Many of the dance venues just sell tickets and do not register the names of the customers,” she said. “It is difficult to get a list of customers from them. I hope those who learned of the [cluster outbreak] and remember they had visited the 14 dance premises come forward to get tests.”

Among the new cases were a cleaner at one dance club and a family member of a previously confirmed patient who taught at the Cheung Sha Wan campus of the Hong Kong Institute of Technology. That infection triggered the suspension of face-to-face classes for 20 students.

“There was also another cleaner [earlier] at one of the other dance clubs who was infected,” Chuang said. “This shows the venues are contaminated and we expect more employees to test positive in the coming days.”

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The order for mandatory testing had come too late to break the transmission chain, warned Dr Gabriel Choi Kin, the president of the Hong Kong Medical Association.

Professor Lo Chung-mau, head of the University of Hong Kong-Shenzhen Hospital, was also critical of the screening measures.

“While mandatory might sound strong, it is actually weak,” he said. “People have two days to hand in their samples. In that time they could still be going out and taking public transport.”

Of the latest confirmed infected people, 45 displayed no symptoms. Seven cases were imported, including arrivals from India, Mexico, Turkey, Russia and Pakistan. The city’s tally of confirmed infections stands at 5,628, with 108 related deaths.

Some 11 infections were untraceable, including a 70-year-old woman from Kwai Chung who had performed Cantonese opera with another person at the Leighton Hill Community Hall in Causeway Bay. Chuang said the woman had not worn a mask, but the roughly 10 audience members were seated some distance away and were wearing face coverings.

A student at the Ling Liang Church M.H. Lau Secondary School in Tai Po has also tested positive and the institution ordered to close. A close contact of the pupil attends the SKH Yuen Chen Maun Chen Primary School, which will also temporarily close.

Another confirmed carrier had visited the Kwan Ho Seafood Restaurant in Mei Foo, where she attended a dance with a “substantial number” of other customers. But health authorities had not detected any other related infections.

Secretary for Food and Health Professor Sophia Chan Siu-chee had warned the public to prepare for a rise in new infections after the cluster at the dance clubs emerged.

Health experts have urged the government to step up supervision of people entering the city from abroad and implement dedicated transport to specific hotels for quarantine to minimise the spread of the virus.

According to the Hospital Authority, 164 patients were still being treated in 19 hospitals. Eight were in critical condition, two in serious condition, with the rest stable, it said.