Pro-democracy demonstrators fought running street battles with riot police on Sunday, the 15th weekend of mass protests demanding more autonomy from the central government in Beijing.
Tens of thousands of people streamed down a six-lane roadway in downtown Hong Kong.
Demonstrators hurled petrol bombs into the government headquarters, torching another vehicle carrying a water cannon that was parked inside the fortified compound.
At least one metro station went up in flames by arson and shops were also hit.
Police in Hong Kong have used water cannon and tear gas against protesters throwing petrol bombs and bricks near government offices in the city.
Earlier, hundreds rallied outside the British Consulate, demanding the UK press China to maintain freedoms guaranteed during the 1997 handover.
The months of unrest were sparked by a proposed extradition bill, which would have made it possible for people in Hong Kong to be extradited to mainland China. Critics said they could have faced human rights abuses.