Hong Kong police fired tear gas at protesters gathered outside government headquarters in a major escalation of violence in the city.
Tens of thousands of people had defied a ban to rally for democracy through the streets on Saturday.
A handful of protesters started fires and threw rocks and petrol bombs towards police, who were separated from the crowds by barriers.
Officers fired tear gas and water cannon as the unrest now enters its fifth month.
Since April, millions of people have taken to the streets and police have clashed with small groups of hardcore protesters.
It is the worst crisis since Hong Kong reverted from British to Chinese rule in 1997.
Under the handover agreement, a one country, two systems legal framework was secured for Hong Kong.
It allowed for far greater civil liberties than on the mainland but protesters say they are being eroded as Chinese interference grows.
The unrest was sparked by opposition to a now-suspended bill that would have allowed the extradition of suspects for trial in mainland China.
Demonstrators have five demands, including the complete withdrawal of the extradition bill and the right to elect their own leader.