Hong Kong riot police fired tear gas and water cannon at hardcore pro-democracy protesters hurling rocks and petrol bombs on Sunday, tipping the violence-plagued city back into chaos after a brief lull in clashes.

Tens of thousands of people defied authorities to march through the city in an unsanctioned rally on Sunday, the latest expression of a popular revolt that has raged for the last 99 days.

The rally descended into violence when small groups of hardcore activists — known within the movement as “braves” — attacked the city's main government complex.

Police fired repeated volleys of tear gas and deployed water cannon trucks after Molotov cocktails and rocks were thrown over security barriers surrounding the complex, which has become a frequent flashpoint in the ongoing protests.

Local television networks broadcast footage of protesters tearing down and burning a huge banner celebrating the upcoming 70th anniversary of the founding of communist China -- as well as torching a Chinese flag.

As evening set in, protesters retreated, chased by riot officers and water cannon firing blue-dyed water. Hong Kong's summer of rage was sparked by a now-abandoned plan to allow extraditions to the authoritarian mainland, a move that prompted millions to hit the streets.

As Beijing and local city leaders doubled down, the protests rapidly snowballed into a much wider anti-government movement.

On Sunday, some demonstrators built barricades, set fires and vandalised subway stations, but the crowds avoided further direct clashes and largely ran away when ranks of police officers got close.

Street brawls

Throughout the evening, multiple brawls broke out between political opponents, including in the districts of Fortress Hill and North Point, the latter a bastion of pro-Beijing sentiment.

Witnesses said a group of Beijing supporters armed with poles and improvised weapons attacked people in Fortress Hill, but were then set upon by a larger crowd of anti-government protesters.

Two men were treated by paramedics for bloody wounds and rushed away in ambulances.

“They attacked us today and yesterday, so we chased and attacked them,” a democracy protester, who gave his name as John, told AFP.

Pro-democracy supporters also attacked at least two men in Causeway Bay and further fights took place in North Point, AFP reporters on scene saw.

The clashes ended a relative lull in recent days in the intensity of skirmishes between police and protesters.

The once-stable international hub has been convulsed by weeks of huge, sometimes violent rallies calling for greater democratic freedoms and police accountability.

The movement is the biggest challenge to China's rule since the city was handed back by Britain in 1997 and shows no sign of ending.

Under a deal signed with Britain ahead of the handover, Hong Kong is allowed to keep its unique freedoms for 50 years.

But democracy activists accuse Beijing of reneging on those promises by tightening political control over the semi-autonomous territory and refusing calls for universal suffrage.

Protest outside UK consulate

Earlier on Sunday, protesters rallied outside Britain's consulate in Hong Kong, demanding London do more to protect its former colonial subjects and ramp up pressure on Beijing over sliding freedoms.

Hundreds of demonstrators sang “God Save the Queen” and “Rule Britannia” outside the consulate, waving the Union Jack as well as Hong Kong's colonial-era flags.

Many protesters said Britain was not doing enough to confront Beijing over its tightening grip on the semi-autonomous city.

They also called for Hong Kongers who want to leave the city to be granted citizenship in Britain or other Commonwealth nations.

“At least with the full citizenship, they can protect Hong Kong people from the Chinese government,” protester Anthony Chau told AFP.

Earlier this week, some 130 UK lawmakers signed a joint letter calling for Britain and Commonwealth countries to come up with an “insurance policy” for Hong Kongers to resettle overseas should they wish to.

Leaderless movement

The pro-democracy movement has vowed to continue until key demands are met, including an inquiry into the police, an amnesty for those arrested, and universal suffrage.

City leader Carrie Lam and Beijing have shown little appetite for making concessions.

Lam has said she is willing to hold a dialogue, but protesters must abandon violence first.

But the movement is leaderless — a response to authorities prosecuting high-profile democracy figures from previous protests — making it difficult for Lam to know who to negotiate with.

Online message boards have filled with plans for new rallies and strikes in the coming weeks, fuelled by two key anniversaries.

September 28 is the fifth anniversary of the start of the failed pro-democracy Umbrella Movement protests, while October 1 is the 70th anniversary of the founding of the People's Republic of China.