Hong Kong: Hong Kong riot police fired tear gas and water cannons 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 streets of the city in an unsanctioned rally on Sunday, the latest expression of a popular revolt that has raged for the last 99 days.

But the rally descended into violence when small groups of hardcore activists -- known within the movement as the "braves" -- tried to attack 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.

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, with local leaders and Beijing taking a hard line. 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.

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 of the protest signs accused Britain of not doing enough to confront Beijing over its tightening grip on the semi-autonomous city.

"Sino-British Joint Declaration is VOID," one read, referencing the 1984 agreement that paved the way for the city's handover, a deal that Hong Kongers were given no say over.

"So far, I'm quite disappointed by the fact that the UK hasn't done anything to support us," protester Alex Leung, a recent graduate, told AFP.

Many called for Hong Kongers who want to leave the city to be granted citizenship in Britain or other Commonwealth nations. Some Hong Kongers were given British National Overseas (BNO) passports before the handover, a document that allows holders easy travel to the UK but grants no working or residency rights.

"At least with the full citizenship, they can protect Hong Kong people from the Chinese government," protester Anthony Chau, who holds a BNO passport, 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.

Hong Kong's summer of rage was sparked by plans to allow extraditions to the authoritarian mainland, a move that promoted million to hit the streets. As Beijing and local city leaders doubled down, the movement rapidly snowballed into a much wider anti-government movement.

The widely loathed extradition bill has now been withdrawn. But 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.

"We have a right to go to protests, to the streets, and we have to voice our demands to the government," said a secondary school student at Sunday's unsanctioned rally who gave his name as Alvin. Both Lam and Beijing have shown little appetite for making any further 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 People's Republic of China.