A new law has empowered Hong Kong authorities to lock down parts of the city hit by Covid-19 for up to seven days, while restaurants will stop offering dine-in services at night and gyms and beauty parlours will close from Thursday to combat a worsening fourth wave of the pandemic.

The city confirmed 100 new infections on Tuesday as Chief Executive Carrie Lam Cheng Yuet-ngor said it was time for tougher measures to contain the spread of the coronavirus, citing more than 600 cases recorded over the past week – the majority of which were contracted locally.

Of the new confirmed cases, 95 were locally transmitted and 27 were untraceable. About 70 preliminary cases were also recorded.

“There used to be a thinking that youngsters who got infected would be fine. But this time, the perception has to be changed too. The situation is very worrying,” Lam said.

“All citizens, unless it’s necessary otherwise, should stay home, even for work. Whether it’s for yourself, your family or society, it is necessary to stay home during this period of time to stop all the social activities.”

Referring to a worrying cluster of infections at the Kwai Shing West public housing estate, she spoke of the possibility of sealing off stricken neighbourhoods if necessary to carry out compulsory testing.

Her de facto cabinet, the Executive Council, approved the legal framework on Tuesday to empower the government to take such action through an amendment to the Prevention and Control of Disease (Compulsory Testing for Certain Persons) Regulation. The new powers came into force at midnight, according to the government gazette.

Lockdown orders will last for up to seven days under the new law and offenders will face a maximum of HK$25,000 (US$3,225) in fines and six months’ imprisonment.

The government did not specify how large a cluster would need to be in order to trigger a lockdown. Secretary for Food and Health Professor Sophia Chan Siu-chee said authorities would make risk assessments to determine which areas needed to be sealed off and for how long.

Affected residents would be required to undergo mandatory tests and stay in the designated areas until all results were available.

“We will not use the measure lightly,” she vowed. “If there are high risks that a considerable number of people are confirmed or suspected to be confirmed cases in a place and there are chances many others could be infected, we may consider issuing the order.”

But a lockdown could also be imposed in situations where even though the number of infections was not sizeable, the risk of transmission was deemed to be high, she said. Officers would help send meals and daily necessities to the residents if the order was to last more than 12 hours.

Calls for the extraordinary measure have grown more urgent as the cluster at Block 8 at Kwai Shing West Estate grew to 18 people, and spread from the fifth floor to four more levels.

Speaking before her Exco meeting, Lam conceded the outbreak “fell into the situation” of requiring a lockdown and the government could have done better by speeding up the process of amending the law.

Social-distancing measures would also be strengthened on Thursday, the health minister said. Restaurants must cease dine-in service between 6pm and 5am, with the measure to remain in place for at least 14 days. Tables will be limited to two people and eateries must operate at half capacity.

The government will also close gyms, sports premises and beauty and massage parlours. Authorities have already shut other popular gathering places, such as bathhouses, mahjong parlours, karaoke lounges and game centres.

Health authorities have expressed concern over the growing severity of the fourth wave, noting the city recorded a daily average of 91 cases over the past two weeks, 92 per cent of which were local and more than a quarter untraceable.

Hospital Authority chief executive Dr Tony Ko Pat-sing said 38 people remained in critical condition and 46 were listed as serious cases. Of the critically ill patients, seven were under the age of 60, three of them younger than 50.

More infected people were requiring treatment in intensive care units (ICU) and if beds became full, hospitals would need to support each other by transferring patients, Ko said.

Authorities have already started to reduce non-emergency services, such as surgeries that require ICU beds for recuperation. The number of such major operations would need to be cut by about 30 per cent in a week or two to free up resources, he said.

With Christmas just weeks away, fears are growing over a possible surge in imported cases and tighter quarantine requirements for incoming visitors are expected to be adopted. Arrivals would be required to be tested using a swab sample instead of deep-throat saliva, which could contribute to error if not done properly, Lam said.

Officials are expected to require all arrivals from outside China to take government-arranged transport to designated hotels for 14 days of quarantine. They would need to be retested on the 19th day of their arrival, Lam said, pointing to the incubation period of the coronavirus.

Professor David Hui Shu-cheong, a respiratory medicine expert at Chinese University and an adviser to the government on the pandemic, said the latest measures were a “tough but necessary cure” during the fourth wave of infections.

It might be possible to relax some restrictions on dining hours and the closure of entertainment venues if the daily caseload dropped down to about 10 to 20 by the time the new measures were set to expire on December 23, he said.

“While keeping certain restrictions in place, the government could afford to reopen cinemas for instance, or other sites that have not seen large outbreaks in this current wave,” Hui said.

The new power to lock down areas could be used to carry out testing at previous virus hot spots, such as the Kwai Tsing Port and the Tsz Wan Shan neighbourhood, he suggested.

“This means even if the officials need to bring sleeping bags into the container terminal or similar sites, and other people can’t go into Tsz Wan Shan district, then so be it … of course they won’t use the power lightly,” he said.

Businesses affected by the new social-distancing measures are expecting a drastic drop in revenue and many could be forced to shut.

Simon Wong Ka-wo, president of the Federation of Restaurants and Related Trades, called for the government to offer one-off financial subsidies as soon as possible. The Employment Support Scheme – which helped businesses pay part of staff wages but ended last month – should also be revived, he said.

“Business is going to be very bad for restaurants in Hong Kong,” Wong warned.