HONG KONG (AFP) -- Police fired tear gas and pepper spray in Hong Kong on Sunday as tens of thousands of black-clad protesters flooded into the streets, a week after pro-democracy candidates scored a landslide local election victory.

The rally heralded an end to a rare lull and a return to the large-scale demonstrations that Hong Kongers have staged for nearly six months, fuelled by growing fears that authoritarian China is stamping out the city's liberties.

It also marked a resumption of the increasingly violent confrontations between protesters and police, with officers shooting volleys of tear gas at crowds that included children.

The day's main rally in the Tsim Sha Tsui neighbourhood was one of three called for Sunday, as protesters seek to keep pressure on the government after the November 24 district council elections.

"The government are still not listening to us so the protests will go on, they will not stop," said a 20-year-old student who gave only his surname, Chen.

"It is hard to predict what will happen. But the people are still very angry and want change."

The rally started peacefully, with people flooding to the waterside neighbourhood by ferry and train.

"Never forget why you started," read a banner carried by protesters taking part in the march.

A little girl with her hair in pigtails led chants reiterating the movement's demands for direct elections of the city legislature and leadership.

'We have to keep going'

But as part of the march ran into a phalanx of police, protesters were told to move back and warned they were straying from the permitted route.

First pepper spray and then tear gas was fired at several locations, in one case catching in the crossfire an older man selling drinks and ice creams from a converted moped.

"I've sold all my drinks but it seems that I can't leave now," he told reporters, his eyes streaming.

But he quickly recovered, tucking into a red ice lolly pulled from his stocks.

"I have nothing to be ashamed of or guilty about," he said with a smile.

Police said they used tear gas after some protesters threw "smoke bombs".

Hong Kong's Beijing-backed chief Carrie Lam has offered no additional concessions in the wake of the elections, and protesters said they felt compelled to return to the streets.

"The government has given no real response, it's unacceptable," 19-year-old student Edmund told AFP.

"We have to keep going. We are fighting for our freedom, not just our own freedom but the next generation too. If we give up now all will be lost."

Earlier, a smaller rally marched peacefully to the US consulate to thank American lawmakers for passing legislation backing the protest movement.

The main rally dispersed as night fell but some protesters moved to other parts of the city.

Throughout the evening tear gas was fired in multiple locations as hardcore protesters vandalised businesses seen as being pro-Beijing and threw projectiles at police.

Officers were seen making multiple arrests.

Fears of fresh violence

Months into the huge protests kicked off by opposition to a bill allowing extradition to China, demonstrators still appear to command widespread support -- with the victory of pro-democracy candidates undercutting government claims of a "silent majority" opposed to the movement.

Ahead of Sunday's rallies there were calls from organisers for people to remain "highly restrained", with fears of a return to the violence that has marked more recent months in the political crisis.

Operators of the city's subway -- which has been repeatedly vandalised after stations were closed ahead of protests while police officers were transported to rally sites -- said it would resume full service on Monday.

But flare-ups of violence remain.

On Saturday night a video in Mongkok district showed a protester brutally assaulting a man as he attempted to clear a barricade.

A voice in the video mocks the man, who stumbled and slumped over after being hit across the head with a heavy metal object.

Police confirmed the incident in a statement: "So far no one has been arrested. The victim suffered a serious head injury and was sent to hospital."

In a radio interview on Sunday morning, police chief Tang Ping-keung slammed the violence.

"It could have killed him," Tang said. (AFP)