Police have fired chemical-laced blue water and tear gas at protesters who lobbed Molotov cocktails outside the Hong Kong government office complex.

Violence flared anew on Sunday after thousands of pro-democracy supporters marched in defiance of a police ban.

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 more than a kilometre to the central business district.

Some waved US and British flags, while others carried posters reiterating their calls for democratic reforms.

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.

Protester Winnie Leung, 50, said: “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.”

The march disrupted traffic and many shops, including the Sogo department store, closed their doors.

Protesters burned Chinese flags and tore down banners congratulating China’s ruling Communist Party, which will celebrate its 70th year in power on October 1.

In familiar scenes, some protesters smashed glass windows and surveillance cameras at a subway station exit.

Hundreds of protesters later targeted the government office complex, throwing bricks and petrol bombs through police barriers.

Police responded by firing volleys of tear gas and using water cannon lorries to spray chemical-laced water as well as blue liquid that helped them identify offenders, in a repeat of confrontational scenes from the last several weeks of the protests.

Protesters retreated but regrouped in the nearby Wan Chai neighbourhood, setting fires outside a subway station exit and on the streets.

They fled again after riot police advanced and the cat-and-mouse battles went on for a few hours before calm returned.

Police fired tear gas again later in the nearby North Point area after protesters obstructed traffic after brawling there earlier with pro-government supporters.

Hospital authorities said eight people were injured throughout the day, including three in serious condition.

Earlier on 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.

-AAP