Hong Kong riot police fired tear gas and water cannons on Sunday at hardcore pro-democracy protesters hurling rocks and petrol bombs, in a return to the political chaos plaguing the city.

Protesters call on UK to protect Hong Kongers from China

Fights break out as Hong Kong polarisation deepens

Tens of thousands of people defied authorities to march through the streets of the financial hub in an unsanctioned rally on Sunday.

But it descended into violence outside the city government's offices in the late afternoon as police battled small groups of radical protesters.

A protester throws a molotov cocktail over a barricade outside the government headquarters in Hong Kong. AFP

Earlier, thousands of demonstrators chanted slogans and marched through downtown Hong Kong on Sunday in defiance of a police ban, as shops shuttered amid fears of renewed violence in the semiautonomous Chinese territory that has been unsettled by months of protests.

A mixed crowd of hardcore protesters in black and wearing masks, along with families with children, spilled into the roads of the Causeway Bay shopping belt and marched for over 2 kilometres (1.2 miles) to the central business district. Some waved US and British flags, while others carried posters reiterating their calls for democratic reforms.

Pro-democracy protesters react as police fire water cannons in Hong Kong on Sunday. Nicolas Asfouri/AFP

Police had turned down a request by the Civil Human Rights Front to hold the march, but the demonstrators were undeterred, as they have been all summer.

"I feel this is our duty. The government wants to block us with the ban, but I want to say that the people will not be afraid," said one protester, Winnie Leung, 50.

The march disrupted traffic and many shops, including the Sogo department store, closed their doors. Some protesters used traffic cones, metal fencing and rubbish bins to set up road barriers.

Protesters burned Chinese flags and tore down banners congratulating China's ruling Communist Party, which will celebrate its 70th year in power on Oct. 1. In familiar scenes, others smashed glass windows and surveillance cameras at a subway station exit.

Police warned in a statement that the assembly was illegal and urged protesters to "stop their illegal acts." Police erected barriers outside the government office complex and water cannon trucks were seen stationed outside the Beijing liaison office.

Anti-government protesters attend a demonstration at Causeway Bay in Hong Kong. Athit Perawongmetha/Reuters

The protests were triggered in June by an extradition bill that many saw as an example of China's increasing intrusion and at chipping away at Hong Kong residents' freedoms and rights, many of which are not accorded to people in mainland China.

Hong Kong's government promised this month to withdraw the bill, which would have allowed some criminal suspects to be sent to mainland China for trial, but protesters have widened their demands to include direct elections for the city's leaders and police accountability.

There have been increasing clashes between protesters and police, who demonstrators have accused of abuses. More than 1,300 people have been arrested since the protests started.

The unrest has battered Hong Kong's economy, which was already reeling from the US-China trade war. It is also seen as an embarrassment to Beijing, which has accused foreign powers of fomenting the unrest.

Anti-government protesters attend a demonstration at Causeway Bay in Hong Kong. Reuters

Earlier Sunday, hundreds of protesters waved British flags, sang "God Save the Queen" and chanted "UK save Hong Kong" outside the British Consulate as they stepped up calls for international support for their campaign.

With banners declaring "one country, two systems is dead," they repeated calls for Hong Kong's former colonial ruler to ensure the city's autonomy is upheld under agreements made when it ceded power to China in 1997.

Demonstrators held similar rallies Sept.1 at the British facility and last weekend at the US Consulate.

Police also banned a planned Civil Human Rights Front march on Aug. 31, but protesters turned up anyway. Clashes erupted that night, with police storming a subway car and hitting passengers with batons and pepper spray.

A protester pours water over a tear gas canister fired by police in Hong Kong. AFP

On Saturday, pro-democracy protesters and supporters of the central government in Beijing clashed at a Hong Kong shopping mall and several public places. Police arrested more than a dozen people and hospital authorities said 25 were injured.

The clashes amid the mid-autumn festival holiday came after several nights of peaceful rallies that featured protesters belting out a new protest song in mass singing at shopping malls. Thousands of people also carried lanterns with pro-democracy messages in public areas and formed illuminated human chains on two of the city's peaks on Friday night to mark the major Chinese festival.

Agencies