The violence broke out after thousands of pro-democracy demonstrators marched in defiance of a police ban.
Earlier hundreds rallied outside the British Consulate, demanding the UK press China to maintain freedoms guaranteed during the 1997 handover.
Months of unrest were sparked by a now-scrapped extradition bill.
It would have made it possible for people in Hong Kong to be extradited to mainland China, where critics say they could face human rights abuses.