Thousands of pro-democracy protesters defied a police ban and began marching through Hong Kong on Sunday, a day after riot police fired rubber bullets and tear gas in the latest violent confrontation to plunge the financial hub deeper into crisis.

Huge crowds gathered in the heart of the city's commercial district on Sunday afternoon, after police gave permission for a static protest in a park but banned a proposed march through the city.

Yet protesters soon spilled into the streets outside the park and began marching in spite of the ban, ratcheting up the likelihood of renewed clashes.

"I feel so conflicted, seeing young people sacrifice their future for Hong Kong," a 22-year-old student protester called Marcus told AFP, breaking into tears.

The latest march comes a day after a town near the border with mainland China descended into chaos, as police battled protesters holding another banned rally against suspected pro-government triad gangs who beat up democracy demonstrators there last weekend.

Riot police used tear gas throughout the afternoon and evening in Yuen Long after tense standoffs with protesters, some of whom were throwing projectiles and had surrounded a police van.

Rubber bullets were fired later in the clashes which ended when officers baton-charged the last remaining demonstrators inside the town's metro station, leaving pools of blood in the same concourse where the suspected triads had attacked the previous weekend.

Saturday's violence compounds the political crisis engulfing the city's pro-Beijing leadership who are seemingly unable, or unwilling, to end the chaos.

Weeks of chaos

Semi-autonomous Hong Kong has been plunged into its worst crisis in recent history after millions of demonstrators took to the streets and sporadic violent confrontations erupted between police and pockets of hardcore protesters.

The demonstrations over the last seven weeks were triggered by a controversial bill which would have allowed extraditions to mainland China, but have evolved into a call for wider democratic reforms and a halt to sliding freedoms.

Yet the unprecedented protests with huge turnouts -- as well as frequent clashes and the sacking of parliament -- have had little luck persuading Beijing or Hong Kong's leaders.

Beijing has issued increasingly shrill condemnations in the last two weeks, but has left it to the city's government to deal with the situation.

City leader Carrie Lam has shown no sign of backing down beyond agreeing to suspend the extradition bill.

Her administration has faced down weeks of public anger and she has made few public appearances beyond visiting injured officers and holding a handful of press conferences.

Battle for Yuen Long

Tensions were significantly raised after last weekend's attack by a pro-government mob in Yuen Long.

The town is in Hong Kong's rural New Territories where many of the surrounding villages are known for triad connections and their staunch support for the pro-Beijing establishment.

That brazen assault hospitalised at least 45 people. Police were heavily criticised for being too slow to respond to the violence, fuelling accusations of collusion or turning a blind eye to the pro-government mob -- allegations the force has denied.

In a rare move, police banned Saturday's rally saying they feared reprisal attacks against villagers from protesters, a decision that only heightened anger towards a force already perceived to be protecting pro-government aggressors.

Tens of thousands of people defied the ban on Saturday and began a peaceful rally.

But small groups of more hardcore protesters, many in helmets and carrying shields, confronted police outside the villages and accused them of protecting triads.

Tensions quickly rose and a now-familiar pattern of running battles between police and protesters began.

On one road, a parked Lexus that was found to have wooden sticks and clubs similar to those used in Sunday's attacks inside it -- as well as a Samurai-style sword -- had been trashed.

By late evening only some 200 protesters remained around the main station who were then charged by riot police.

Police on Sunday said 13 arrests were made in Yuen Long.

Among them was Max Chung, a young activist who had initially applied for permission to hold the Yuen Long protest. He was arrested for inciting an illegal assembly.

Hospital authorities said 24 people were injured, most of whom have been discharged but with two remaining in a serious condition.