Riot police fired tear gas at pro-democracy protesters in Hong Kong Sunday evening during a second consecutive night of clashes, as China delivered fresh warnings over the unrest battering the city.

The semi-autonomous southern Chinese city is reeling from two months of protests and violence triggered by opposition to a planned extradition law that has evolved into a wider movement for democratic reforms.

In the latest unrest, police fired brief volleys of tear gas at protesters who were approaching their lines in the well-heeled Sheung Wan district.

There were also stand-offs between protesters and riot police close to the Liaison Office, the department that represents Beijing in the city.

Two weekends ago, the office was pelted with eggs and paint in a move that infuriated Beijing and sparked increasingly shrill warnings from the mainland.

The latest clashes came as China's official Xinhua news agency published a new commentary on Sunday saying “ugly forces” were threatening the country's “bottom line”.

“The central government will not sit idly by and let this situation continue,” the agency wrote.

Hong Kong's protests constitute the most significant popular revolt in decades, directly challenging Beijing's rule as well as channelling rage at both the local leaders and police.

But they have had little luck persuading their opponents who have only hardened their stance.

More than 200 protesters have been arrested — dozens charged with rioting — while the Chinese military has said it is ready to quell the “intolerable” unrest if requested.

Violence escalates

The last fortnight has seen a surge in violence on both sides with police repeatedly firing rubber bullets and tear gas to disperse increasingly hostile projectile-throwing crowds.

This weekend proved little different.

On Saturday evening police fought hours-long battles with small groups of hardcore protesters in Tsim Sha Tsui — a harbourside district known for its luxury malls and hotels — after they besieged a police station and used a giant slingshot to fire bricks at the building.

Acrid plumes of tear gas filled streets usually teeming with tourists and shoppers.

Further clashes occurred in Wong Tai Sin, a nearby working-class district where large crowds of angry residents joined protesters.

On Sunday afternoon, protesters held two large rallies in Tseung Kwan O and Kennedy Town that began peacefully but soon descended into violence as protesters took over nearby roads.

In Tseung Kwan O, small groups of masked protesters threw bricks and eggs at the local police station, smashing multiple windows.

Protesters vowed to keep hitting the streets.

“I'm more worried than hopeful,” Florence Tung, a 22-year-old trainee lawyer who was among the thousands marching through Tseung Kwan O, told AFP.

“It's like no matter how much us citizens do, we cannot change the government,” she added, referring to the city's unelected pro-Beijing leaders.

Kai Hou, a 41-year-old education worker, said he disagreed with the tactics of more hardcore violent protesters but supported their overall goals.

“Not everyone may approve of their radical acts, but their goal is simple, they want to build a better Hong Kong,” he told AFP.

Sliding freedoms

Under the terms of the 1997 handover deal with Britain, Hong Kong has rights and liberties unseen on the Chinese mainland, including an independent judiciary and freedom of speech.

But many say those rights are being curtailed, citing the disappearance into mainland custody of dissident booksellers, the disqualification of prominent politicians and the jailing of pro-democracy protest leaders.

Public anger has been compounded by rising inequality and the perception that the city's distinct language and culture are being threatened by ever-closer integration with the Chinese mainland.

Hong Kong chief executive Carrie Lam has made few concessions beyond agreeing to suspend the extradition bill, and has shied away from public appearances.

Protesters are demanding her resignation, an independent inquiry into police tactics, an amnesty for those arrested, a permanent withdrawal of the bill, and the right to elect their leaders.

Protesters hope to ramp up pressure on Lam by launching a city-wide strike on Monday as well as seven simultaneous rallies, a feat that would be a rare accomplishment in a freewheeling finance hub where unions have little sway.

The strike action appears to be gaining more traction than previous walkouts in the last few weeks as the civil disobedience campaign intensifies.