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 "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.

Local television networks broadcast footage ofprotesters tearing down and burning a banner celebrating the upcoming 70th anniversary of the founding of communist China — as well as burning a Chinese flag. As evening set in, protesters retreated, chased by riot officers and water cannon firing blue-dyed water.

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.

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.