The Civil Human Rights Front said the force had rejected its applications for rallies on July 1, the 23rd anniversary of the former British colony's handover to China.
The Hong Kong police department on Saturday has denied permission for an annual march in the former British colony on July 1 to mark the anniversary of the city's 1997 return to China, the organiser and the police said in separate statements.
In a notice to the organiser, Civil Human Rights Front (CHRF), the police cited the city's current rules limiting gatherings to no more than 50 people due to the coronavirus pandemic, saying public assemblies and demonstrations are not exempted.
The notice, posted on the front's Facebook page, also cited violent incidents that took place after public assemblies and rallies organised by the group over the past 12 months.
"Upon risk assessment, police considered that the public meetings and procession are high-risk activities," the local police department said in a separate post on the department's Facebook page.
The move comes ahead of a three-day meeting from Sunday of China's parliament, which is expected to enact new national security legislation on the eve of the 23rd anniversary of Hong Kong's return to Chinese rule.
China says the national security law, which has alarmed foreign governments and activists will target only a small group of troublemakers and people who abide by the legislation have no reason to worry.
The critics fear that this new legislation would smother the financial hub's treasured freedoms.
It is the first time the annual march has been banned since the CHRF began them in 2003 when half a million people came out to protest against the local government's attempt to introduce a national security law.
That bill was shelved, but Beijing has decided to impose a new security law in the city, claiming Hong Kong protesters were involved in separatism.
China took back control of Hong Kong on July 1, 1997, under a "one country, two systems" formula that allows freedoms not enjoyed in mainland China, including freedom of protest and a much-cherished independent judiciary.
In recent years, the anniversary of the handover has been marked by deepening despondency about what many residents saw as increasing meddling by the mainland and the erosion of freedoms.
The semi-autonomous city has been convulsed by a year of huge and often violent rallies that began with an eventually aborted criminal extradition bill but morphed into a popular call for democracy and police accountability.
US President Donald Trump's administration said on Friday it was restricting visas for a number of Chinese officials for infringing on the autonomy of Hong Kong, as Congress seeks tougher sanctions.
Announcing the visa restrictions, US Secretary of State Mike Pompeo did not name any of those targeted.
The Chinese embassy in Washington said, "no one has any legal grounds or right to make irresponsible comments on Hong Kong affairs".
The EU also warned China it would face "very negative consequences" if it pressed ahead with the new bill.