Police on Saturday cited risks to public health and the potential for violence as they refused permission for a July 1 pro-democracy march.
The Civil Human Rights Front had applied for authorisation to hold its normally annual march from Causeway Bay's Victoria Park to the government's headquarters in Admiralty.
But in a letter to the front, the force cited the city's current gathering limit of 50 people due to the coronavirus pandemic, saying demonstrations could increase the risk of the virus spreading.
It also said past demonstrations organised by the front were marred by violent incidents, and there was reason to believe that some participants would deviate from the planned route and take part in violent activities.
The front said it would appeal against the decision.
The group's vice-convenor Eric Lai said the police's decision wasn't surprising, and the reasons for the rejection were clearly political.
"We knew the police would object to our protest because it's quite obvious that the government wants to present a false appearance on the first of July after the proposed passage of the national security law," Lai said.
"They don't want to have a strong majority of citizens on the streets opposing the national security law," he told RTHK's Richard Pyne.
Lai added that the concerns cited over coronavirus were not consistent with the government's decision to open Ocean Park recently.