Hong Kong police officers have pulled their guns and one fired a warning shot after they were attacked by protesters with sticks and rods.

Key points: After hoisting warning flags, police fired water cannon and tear gas at protesters

After hoisting warning flags, police fired water cannon and tear gas at protesters Demonstrators responded by hurling bricks and petrol bombs

Demonstrators responded by hurling bricks and petrol bombs Beijing has sent a clear warning that forceful intervention is possible

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The protesters called the police "gangsters" as they chased them on Sunday night following a standoff earlier in the evening.

Police said in a statement six officers drew their pistols and one officer fired a warning shot into the air.

Earlier, violence escalated after tens of thousands of anti-government protesters marched in torrential rain, regrouping under a sea of umbrellas after clashes a day earlier that prompted police to fire tear gas for the first time in more than a week.

While a large crowd rallied in a nearby park, another group of protesters took over a main street, strewing bamboo poles on the pavement and lining up orange and white traffic barriers and cones to try to obstruct the police.

After hoisting warning flags, police used tear gas to try to disperse the crowd.

Protesters responded by throwing bricks and petrol bombs toward the police.

The result was a scene of small fires and scattered paving bricks on the street between the two, rising clouds of tear gas and green and blue laser lights pointed by the protesters at the police.

The Chinese-ruled city's rail operator, MTR Corp, suspended some services to try to prevent people gathering but protesters made it to a sports stadium in the vast container port of Kwai Chung, from where they were marching to nearby Tsuen Wan.

Demonstrators stand behind a barricade during Sunday's protest in Tsuen Wan, in Hong Kong's Western New Territories. ( Reuters: Kai Pfaffenbach )

The latest skirmish comes after Saturday's march in the industrial district of Kwun Tong, which saw protesters build bamboo barricades and set fires in the streets.

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Police said they arrested 29 people, ranging from 17 to 52 years old, for various offenses including unlawful assembly, possession of offensive weapons and assaulting police officers.

The clashes, while not as prolonged or violent as some earlier ones, ended a brief lull in the violence.

Police used tear gas after some protesters threw Molotov cocktails and bricks and others tore up "smart" lamp posts equipped with surveillance cameras.

The protests had turned largely peaceful the previous weekend, after weeks of escalating violence and multiple closures of the financial hub's busy international airport.

M Sung, a 53-year-old software engineer in a black mask emblematic of the many older, middle-class citizens at the march, said he had been at almost every protest and would keep coming.

"We know this is the last chance to fight for 'one country, two systems', otherwise the Chinese Communist Party will penetrate our home city and control everything," he said.

"If we keep a strong mind, we can sustain this movement for justice and democracy. It won't die," Mr Sung said.

Protesters say they are fighting the erosion of the "one country, two systems" arrangement under which the former British colony returned to China in 1997 with the promise of continued freedoms not enjoyed on the mainland.

Tear gas and flames engulfed a road in Kowloon as protesters skirmished with police ( Reuters: Kai Pfaffenbach )

While a large crowd rallied in a nearby park, another group of protesters took over a main street, clashing with police. ( Reuters: Kai Pfaffenbach )

The protests, which started over a now-suspended extradition bill and evolved into demands for greater democracy, have rocked Hong Kong for three months and plunged the city into its biggest political crisis since the handover.

They also pose a direct challenge for Communist Party leaders in Beijing, who are eager to quell the unrest ahead of the 70th anniversary of the founding of the People's Republic of China on October 1.

Beijing has sent a clear warning that forceful intervention is possible, with paramilitary forces holding drills just over the border in Shenzhen.

"Some radical protesters have removed railings … and set up barricades with water-filled barriers, bamboo sticks, traffic cones and other objects," the police said in a statement.

"Such acts neglect the safety of citizens and road users, paralysing traffic in the vicinity."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 16 seconds 16 s Hong Kong protesters fell 'smart' lampposts

Transport to the airport appeared normal on Sunday, despite protesters' plans for a day-long "stress test" of transport in the international aviation and financial hub.

More than 700 people have been arrested since the demonstrations began in June.

Meanwhile, the neighbouring gambling territory of Macau, a former Portuguese colony that returned to Chinese rule in 1999, elected former legislature head Ho Iat Seng as its leader on Sunday — the sole approved candidate.

Ho, who has deep ties to China, is expected to cement Beijing's control over the "special administrative region", the same status given to Hong Kong, and distance it from the unrest there.

Tens of thousands of pro-democracy demonstrators hit the streets of Kwai Fong in Hong Kong in the pouring rain on Sunday. ( Reuters: Kai Pfaffenbach )

ABC/Wires