Hong Kong's pro-democracy protesters returned to the streets on Sunday for a series of marches and rallies after a rare period of calm in nearly six months of unrest.

Sunday's demonstrations come after brief skirmishes erupted overnight, with a man assaulted as he tried to clear barricades and police firing tear gas for the first time since November 24 district council elections that saw pro-democracy candidates win a landslide.

Three events are planned for Sunday, including a march to the US consulate to thank American leaders for legislation backing the city's protest movement.

An evening march will reiterate the movement's five demands, which include direct elections for the city's legislature and leadership, and a probe into alleged police brutality against demonstrators.

Skirmishes erupted overnight between protesters and police in Hong Kong after a rare period of calm in the city
Skirmishes erupted overnight between protesters and police in Hong Kong after a rare period of calm in the city AFP / Nicolas ASFOURI

"I just want to remind everyone that despite the small victory in district council election, we must not forget why we started all this and we must return to our main theme -- reclaim Hong Kong, revolution of our times; five demands, not one less," an organiser of the evening rally wrote on the Reddit-like LIHKG forum.

Police have issued permits called "letters of no objection" for all three events, including a morning rally for children and seniors which went ahead without incident, and the post urged people to remain peaceful.

"I want to beg everyone to remain highly restrained during the hours covered by the letter of no objection. Otherwise I could be charged with inciting riots," the organiser wrote in the anonymously penned post.

Three events are planned for Sunday, including a march to the US consulate to thank American leaders for legislation backing the city's protest movement
Three events are planned for Sunday, including a march to the US consulate to thank American leaders for legislation backing the city's protest movement AFP / Anthony WALLACE

On Saturday night, police fired three rounds of tear gas for the first time since the elections, after protesters blocked roads in the Mong Kok neighbourhood.

Police have fired more than 12,000 tear gas canisters since the protests began, with gas masks becoming a must-have for those attending demonstrations but often a cause for arrest if discovered.

A video that emerged online also showed a protester brutally assaulting a man as he attempted to clear a barricade.

The commentator mocks the man, who stumbles and then slumps over after being hit across the head with a heavy metal object, blood trickling from a wound.

In a radio interview on Sunday morning, police chief Tang Ping-keung said an attack took place in Mong Kok.

"It could have killed him," Tang said.

A police source confirmed that the incident seen in the video occurred in Mong Kok on Saturday night and that it was under investigation.

The condition of the man was not immediately clear. But hospital authorities said three people were brought in on Saturday night with protest-related injuries, one of whom was discharged while the other two were in a stable condition.

Protests erupted in Hong Kong nearly six months ago in opposition to a bill that would have allowed extraditions to mainland China.

The bill has been withdrawn, but the movement has snowballed into wider calls to protect the city's freedoms.