Pro-democracy Cantopop star Denise Ho had a forthcoming concert canceled at the last minute by the venue on Friday, amid a city-wide crackdown on public dissent and political opposition under a draconian national security law imposed by Beijing.

"We regret to announce that the Hong Kong Arts Centre (HKAC) has informed us about the abrupt cancellation of our approved booking for Denise Ho’s performance "Hocc Shouson Live 2021" at the Shouson Theatre," Ho's official Twitter account said on Wednesday.

Ho, an outspoken supporter of Hong Kong's 2019 protest movement, was to have performed there over four nights from Sept. 8-12.

A letter from the HKAC informing Ho of the decision cited a clause allowing performances to be canceled if "public order or public safety would be endangered during the course of the performance," according to a screenshot of the letter posted to Ho's Twitter account.

Ho later commented on her Facebook page: "Thanks, everyone, I'm OK, holding up. In a city where they can't even allow a concert to go ahead, I will have to sing out, and sing louder."

She said a single concert would be livestreamed on Sept. 12 instead.

The cancelation came after the Hong Kong Arts Development Council warned in March that the city's formerly vibrant arts sector would soon face far greater scrutiny under the national security law, which forbids public criticism of the city’s government or the ruling Chinese Communist Party (CCP).

It warned that artists, exhibits, or projects using its funding must comply with Hong Kong law, including the national security law, which took effect on July 1, 2020.

A spokesman for the Hong Kong Democratic Party described the decision to pull the plug on Ho's gigs as "inexplicable."

"If no further explanations are forthcoming, then it is tantamount to unreasonable censorship of a performance," Chan Po-ming said.

He said that tickets to the shows had sold out and yet the performances had been canceled with less than one week's notice.

"This violates the spirit of the contract," Chan said, calling on the HKAC to give a detailed account of its reasoning as soon as possible.

Civic groups harassed

Meanwhile, Hong Kong police wrote to two civic groups requiring them to supply details of their members, activities, and sources of funding under the national security law, according to local media reports.

"RTHK understands that the 612 Humanitarian Relief Fund and the Alliance for True Democracy have been asked to provide information before a deadline," the government broadcaster reported on Wednesday.

It cited sources as saying the request was made under a court warrant in accordance to the national security law, adding that the information sought include details on donors and recipients, its crowdfunding campaign, and how the money was used by the organizations.

Police confirmed in a statement that the 612 Humanitarian Relief Fund is being investigated, but gave no further details.

The move came one day after the fund -- which offers assistance and legal support to protesters arrested during the 2019 protest movement -- said it would stop accepting new funding applications, and announced plans to wind down its operations "in an orderly manner."

Hong Kong is undergoing an unprecedented emigration wave in the wake of the national security law, which has landed dozens of former lawmakers and opposition activists behind bars awaiting trial for "subversion" after they took part in a democratic primary to select the best candidates in elections to the Legislative Council (LegCo).

'Patriotic education'

The government has also used the law to mandate patriotic education in the city's schools, abolishing Liberal Studies, a critical thinking program blamed by the CCP for inspiring several waves of mass popular protest over eroding freedoms in recent years.

There are concerns over falling student enrolments as parents take their children out of schools before leaving the city for good, as well as a drain of qualified teachers.

The city's education bureau has already issued teaching certificates to more than 1,500 unqualified teachers to plug the shortfall in schools.

A former Chinese-language teacher who gave only the name Alan said he brought forward his plans to leave the city to April 2021 after the arrests of the pro-democracy politicians for subversion.

"I used to play news clips from the [now-shuttered] i-Cable China news team ... so we could discuss some of the issues affecting China's development," Alan said.

"But since last year, it has become increasingly clear that such teaching methods and materials can no longer be used," he said.

"Parents or students could say that you are biased."