Hong Kong police and protesters exchanged tear gas and petrol bombs as an illegal anti-government march that attracted tens of thousands descended into chaos, with hundreds of shops trashed and Chinese banks and metro stations targeted.

After two weeks of relative calm, the major rally showed that the pro-democracy campaign had not lost support and that hardcore protesters will continue to clash with police.

Protesters dressed in black erected fiery barriers on Nathan Road, a major retail strip in the Kowloon district, as scores of riot police, shields in front, marched towards them, while others fired tear gas.

Protesters earlier threw petrol bombs at the nearby Tsim Sha Tsui police station after police inside fired volleys of tear gas to disperse protesters on the street.

A police water canon truck sprayed jets of blue-dye into the crowd at the police station, sending hundreds fleeing. Police have used the blue dye to identify protesters.

Dozens of riot police vans then descended on Nathan Road, along with water canon trucks. It was the heaviest use of water canons by police and many people hit with the water developed coughs, suggesting an irritant may be mixed with the water.

Police used trucks to smash through protest barriers.

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

Hong Kong has been battered by months of often massive and violent protests over concerns that Beijing is tightening its grip on the city, the worst political crisis since Britain handed the city back to China in 1997.

The protests in the Chinese-ruled city also pose the biggest popular challenge to China's President Xi Jinping since he took power. Beijing has denied eroding Hong Kong's freedoms and Xi has vowed to crush any attempt to split China.

The unrest was sparked 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.

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

"HONGKONGERS RESIST"

Police had declared Sunday's march illegal due to concerns over public safety. Protesters, ranging from young students to the elderly, many carrying umbrellas to shield their faces from street surveillance cameras, face arrest.

For the first few hours, the march was peaceful.

At the start of the march banners reading "Free Hong Kong" stretched across the ground. Other posters read "HongKongers Resist," while graffiti on one wall said "Better Dead than Red."

Hardcore protesters, who have staged months of running battles with police, set up road blocks and sprayed graffiti saying: "We chose to die on our feet rather than live on our knees!." Some tore up pavement bricks for clashes with police.

Protesters believe the police refusal to issue a permit for Sunday's march was an attempt to limit their numbers, as some would fear being arrested.

"The government pretends we just want to destroy the city. We’ll be out for as long as it takes to let the world know it is them who are destroying it," said Ray, 24, who planned to go home after a few hours as he feared arrest. Like most protesters, he did not want to give his full name.

Hong Kong is governed under a "one country, two systems" formula, which permits the city freedoms not available on the mainland such as an independent judiciary.

Protesters are angry at Hong Kong's leader Carrie Lam for what they see as her failure to protect those freedoms from an encroaching Beijing, imposing colonial-era emergency powers, and allowing what they say is excessive force by police.

"Carrie Lam is not listening to us at all. This may work in China but not in Hong Kong," said Cheung, a 33-year-old woman wearing a face mask and black Tshirt, symbols of the democracy movement.

"You can't ask a city that already has freedom to walk backward. You can't do this in an international city," she said, adding she was not afraid of being arrested.

Sunday's violence comes after Lam's annual policy address last Wednesday failed to address protesters' demands.

Protesters have 5 core demands: universal suffrage, an independent inquiry into police action against protesters, amnesty for those charged, and an end to describing protesters as rioters, and the formal withdrawal of the extradition bill.

Lam has said the bill is dead, but it is yet to be formally withdrawn. She has rejected the other demands. On Sunday she said a police complaints inquiry will be completed before the end of the year.

Two people have been shot and wounded by police and thousands injured since the protests escalated in June. Police have arrested more than 2,300 people.

The Asian financial hub is facing its first recession in a decade because of the unrest, with retail and tourism badly hurt. On Sunday shops, both luxury and small, were closed along the march route.

The city's metro, which carries some 5 million people daily, will again shut early.