The statement came after a largely peaceful but illegal march on Hong Kong Island abruptly ended on Sunday when clashes broke out between protesters, Beijing supporters and police.

The letter, submitted by the Junior Police Officers' Association, stated that when rioters engage in "potentially deadly attacks," officers are required to respond with "corresponding force or weapons to stop it, including live ammunition."

Protesters set fire to barricades late Sunday and violent pro-China activists attacked their black-clad opponents in transport stations. In one instance, officers drew their guns as a warning for protesters who threw Molotov cocktails from close range.

In 15 weeks of citywide protests, police have employed crowd dispersal tactics including tear gas, rubber bullets, bean-bag rounds and water cannon that emits blue dye. Last week, off-duty officers were given "blanket approval" to carry out arrests and use force when necessary.

Chief Executive Carrie Lam made a major concession to protesters, offering to withdraw the legislative bill that ignited the protest movement, but she has failed to meet any other demands including electoral reform and an independent inquiry into police violence against protesters.

The legislative bill would have allowed for residents to be extradited to mainland China, stoking fears that the former British colony was losing its autonomy, promised until 2047 under China's "one country, two systems" agreement with Britain.

Australian Associated Press