Tear gas and petrol bombs have reignited anger and fear on the streets of Hong Kong as a massive pro-democracy march descended into chaos and violence.

It had been relatively calm in the region for the past two weeks following four months of unrest. But in recent days there were signs that tensions would boil over once more after Chinese President Xi Jinping threatened protesters would be “crushed” and their bones “shattered”. 

Sure enough, by nightfall on Sunday the streets of Hong Kong were once again filled with tear gas. Violence on both sides suggested authorities and hardcore rally participants are prepared to escalate their tactics.

A bystander is treated after police fired tear gas. Photo: Getty

The march started off peacefully, with footage showing tens of thousands of people slowly strolling side by side through city streets in solidarity against heavy-handed police tactics and bans on face masks. By late night, however, hundreds of businesses were trashed and metro train stations torched by masked demonstrators who lobbed petrol bombs at banks and other businesses linked to the Chinese mainland.

At least one police station was set alight in retaliation to police using tear gas and water cannons to disperse crowds. Early on Monday morning police said they had discovered a suspected explosive device inside a cardboard box.

With demonstrators continuing to defy a ban on facial coverings brought in under emergency law, authorities appeared to have found a new tactic to identify masked march participants; they sprayed jets of blue dye that would stain clothes and skin.

Reporters on the ground said it was the heaviest use of water jets by police since the protests began. Many people hit with the water developed coughs, suggesting an irritant may have been mixed with the water.

A protester pours fuel on a fire at a metro station. Photo: Getty

Authorities had earlier declared the rally illegal, only further angering demonstrators who are fighting against what they see as an erosion of democracy.

During the last unauthorised protest on October 1 – the 70th anniversary of the People’s Republic of China – a student was shot in the shoulder after police fired live ammunition on the streets.

It marked the first use of lethal force since the start of protests four months ago.

Violence escalated again on Wednesday last week when Jimmy Sham, a leading figure of the pro-democracy movement, was brutally bashed by a group of unidentified men wielding hammers.

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Protesters have remained resolute in the face of the violence, however, vowing to continue their fight. 

“You can see Hong Kongers won’t easily give up their right to demonstrate … today’s turnout is more than I expected,” protester Daniel Yeung said on Sunday.

“You can see that as long as people keep coming out in large numbers we are safe and can keep fighting.”

Protesters threw petrol bombs at the Tsim Sha Tsui police station on Kowloon peninsula after police inside fired volleys of tear gas to disperse demonstrators on the street.

A protester, dressed all in black, throws a petrol bomb. Photo: Getty

Some erected fiery barriers on Nathan Rd, a major retail strip in the Kowloon district, as riot police, shields in front, marched towards them, while others fired tear gas.

As riot police advanced protesters fell back to their next barricade, unlike past rallies when they stood and clashed with police, throwing petrol bombs and bricks.

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Along the march route, protesters torched and trashed metro stations and hundreds of shops, throwing goods onto the streets. Several Chinese banks were targeted.

Protesters have in the past targeted Chinese banks and shops with links to mainland China, leaving mainland Chinese living in Hong Kong worried about their safety.

The unrest was sparked four months ago by a bill that would have allowed extradition to mainland China for trial in Communist Party-controlled courts.

It has since widened into a pro-democracy movement.

-with AAP