The Civil Human Rights Front, Hong Kong's largest pro-democracy group, has been denied permission for a march on Sunday against the government's mask ban.
They had also intended to use the event to call for reform of the police force.
In a letter of objection issued on Friday, officers cited violent incidents stemming from recent protests as the reason for banning the Sunday march.
“The commissioner of police thinks there is a need to maintain public safety, public order and protect other persons’ rights and freedoms,” the letter read.
It also noted that protesters had taken home-made explosives, petrol bombs and other weapons to previous protests.
The force has refused three other applications by the front in the past three months, on similar grounds. On all occasions, thousands marched on despite the bans.
The front had applied for approval to march from Salisbury Garden, Tsim Sha Tsui to the high-speed rail terminal in West Kowloon at 1.30pm on Sunday.
Apart from echoing the main demands of the protest movement, which has been running for months, the front also called on the government to abolish the anti-mask law it brought it earlier this month.
The law, enacted without Legislative Council scrutiny under the Emergency Regulations Ordinance, stipulates a penalty of jail for up to one year and a fine up to HK$25,000 (US$3,200) for wearing any face covering at a public gathering.
Chief Executive Carrie Lam Cheng Yuet-ngor said the ban was to deter radical protesters from breaking the law, following increased levels of violence against the police and damage to shops and the city’s railway system.
The front’s vice-convenor Eric Lai Yan-ho said it would likely appeal the objection.
“We are still studying the letter of objection,” Lai said.
The ban came days after the front’s convenor, Jimmy Sham Tsz-kit, was assaulted in Mong Kok by a group armed with hammers on Wednesday night.
Writing on his social media account on Wednesday, Sham called on the public to not harbour hatred against ethnic minorities, after reports that the attackers were not ethnically Chinese.
“The problem is always with the system, with the authorities,” Sham wrote.
As of Friday, Sham was still in Kwong Wah Hospital.