Dozens of senior Hong Kong officials and lawmakers were ordered into quarantine on Friday after a coronavirus cluster detected at a birthday party attended by many of the city's political elite widened.

The high-profile entries into Hong Kong's 21-day quarantine facilities have left city authorities red-faced as they prepare to install a new "patriots only" legislature, which is scheduled to hold its first meeting next week.

Hong Kong leader Carrie Lam has ordered a probe into 13 top officials who attended the party, vowing that "appropriate action" will be taken after an investigation is complete.

The 13 officials -- including the city's police, immigration and anti-corruption chiefs -- were ordered not to work during quarantine, and that days spent in isolation will come out of their holiday allowances.

"Government officials must set an example and uphold discipline, maintain a high level of alertness at all times and avoid participating in high-risk activities," Lam wrote in a late-night statement.

Those senior officials, as well as 19 legislators, were among about 170 people who attended a Monday night birthday bash for Witman Hung, a member of China's top lawmaking body.

A single preliminary coronavirus case was detected at the party, sparking an initial run of quarantine orders on Thursday for a handful of people deemed to be close contacts.

But health inspectors widened the quarantine order on Friday to include all attendees after a second person who was at the event tested preliminary positive for the virus.

"All the people present at the party need to be quarantined," Chuang Shuk-kwan from Hong Kong's Centre for Health Protection told reporters.

The health department did not give a breakdown of the attendees headed to Hong Kong's quarantine camp.

But the government had previously confirmed that 13 senior officials and 19 lawmakers made an appearance at the tapas bar where the birthday bash was held.

"We have more than 60 people at the quarantine centre now and the process is still ongoing," health official Edwin Tsui said.

One of the lawmakers, firebrand nationalist Junius Ho, met China's top official for Hong Kong policy across the border in Shenzhen two days after the party.

While the party was legal under rules at the time, health officials had advised people three days earlier to avoid big gatherings.

Hong Kong authorities tightened Covid-19 restrictions this week after a small outbreak of the Omicron variant was traced to a Cathay Pacific flight crew
Hong Kong authorities tightened Covid-19 restrictions this week after a small outbreak of the Omicron variant was traced to a Cathay Pacific flight crew AFP / Bertha WANG

Lam said she was "very disappointed" that so many officials and cabinet members had joined a large gathering.

Among those in attendance were Hong Kong's top cop Raymond Siu, immigration chief Au Ka-wang, home affairs secretary Caspar Tsui and the head of the city's anti-corruption watchdog.

"I offer my sincere apology to all people of Hong Kong. I have reflected on this incident and shall be more vigilant in future," Au said in a statement Friday.

Lawmakers sent to the quarantine facility will now miss the legislature's first sitting next Wednesday.

"I feel I acted carelessly and I rather regret it," lawmaker Duncan Chiu told local radio. "This has set a bad example."

Multiple local outlets ran a photo showing birthday celebrant Hung singing into a microphone with his arm around the shoulder of a smiling woman, both unmasked.

Hung issued an apology late Thursday and will be among those quarantined.

The president of the Legislative Council, Andrew Leung, said he hoped next week's meeting would continue as planned.

A new group of 90 city lawmakers were selected last month under new "patriots-only" rules imposed by Beijing, which vetted all candidates for their political loyalty.

Only 20 were directly elected amid record low turnout, with the rest selected by reliably pro-Beijing committees.

The government upped coronavirus-related restrictions this week, after an outbreak of the Omicron variant was traced to a member of a Cathay Pacific flight crew.

The city has recorded 223 Omicron cases as of Friday night.

Earlier this week, Lam said Cathay's top brass must take responsibility for employees breaching isolation rules even if "they may not know their employees' every act".

Asked by reporters on Thursday whether she should do the same, Lam said it would be "bad for governance" if she took responsibility for the behaviour of her ministers, who were expected to lead by example.

It is not the first time officials have found themselves in hot water over health rules.

Last year, three immigration officials, including Au, were fined after they broke social distancing rules to eat hotpot at a luxury clubhouse with a mainland property developer.