Chief Secretary Matthew Cheung Kin-chung also refused to exercise his power under the city’s emergency health law to permit the gathering.

The letters seen by the Post said the police had considered social-distancing rules when making the decision and warned residents over taking part in an illegal assembly.

“Though police have met with your representatives on your proposed arrangements for the assembly, police still reasonably believe that such additional conditions do not help maintain public order and protect the rights and freedoms of others,” one letter read.

Secretary for Security John Lee Ka-chiu said on the sidelines of a regular Legislative Council meeting the police made the decision based on the Public Order Ordinance, which gave the commissioner the power to object to gatherings. But he did not mention whether the national security law was a factor involved.

“The commissioner of the police takes into consideration factors that are prevalent,” he said.

Lee refused to be drawn on individual cases, such as whether private commemoration of the crackdown would amount to a breach of national security, but issued a stern warning.

“Anyone who tries to challenge the law, be it the Public Order Ordinance or national security law, police will take action,” he said.

Democratic Party chairman Lo Kin-hei said the June 4 vigil was an indicator of the level of freedom of speech in the city.

“This, I believe, constitutes an affront against people’s freedom to express their opinion,” he said.

The alliance said it would challenge the decision and urged the appeal board to process their application as soon as possible.

“Commemorating June 4 is the collective memory of the Hong Kong people for the past 31 years and the Hong Kong Alliance in Support of Patriotic Democratic Movements of China will continue fighting for the rights of the people to mourn for June 4,” it said.

The group had previously written to Cheung urging him to give the group an exemption as he had the power to green light gatherings “necessary for the government’s operation” or in “exceptional circumstances” which served public interest.

RTHK launches probe over footage of June 4 vigil organisers in show

But the alliance said the Chief Secretary’s Office wrote back to them on Thursday afternoon, saying their demand was not approved.

According to the office, the Covid-19 pandemic remained severe and the number of cases including ones involving mutant strains around the world was continuously rising.

The government did not recommend organising mass gatherings involving a large number of people with prolonged duration of contact, as there would also be difficulty in controlling the crowd size and ensuring adequate social distancing among participants.

“On the basis of the information provided by the said organisation, the government is unable to establish that the proposed organisation of the procession and the gathering would meet the conditions under section 5 of Cap. 599G,” the statement read, referring to the Prevention and Control of Disease (Prohibition on Group Gathering) Regulations.

Hong Kong’s annual Tiananmen Square vigil banned on Covid-19 grounds

The mass gathering in Causeway Bay has been the only large-scale commemoration of the crackdown on Chinese soil.

Police banned the vigil on health grounds for the first time last year, but an estimated 20,000 people turned out at Victoria Park.

A total of 26 leading opposition figures were later arrested and charged, including media tycoon Jimmy Lai Chee-ying and activist Joshua Wong Chi-fung.

Wong and three district councillors pleaded guilty last month to knowingly taking part in an unauthorised assembly, an offence under the Public Order Ordinance that could land them five years in prison.

Last month, the Leisure and Cultural Services Department pointed to the Covid-19 pandemic when it said the alliance’s application for the use of Victoria Park on June 4 would not be processed.