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.
Earlier this month the bill was finally withdrawn - but protesters continue to call for full democracy and an investigation into allegations of police brutality.
Reports say some protesters threw bricks at police outside China's People's Liberation Army base, which is near to the Hong Kong parliament and government offices.
They also set fire to a banner proclaiming the 70th anniversary of the founding of the People's Republic of China, Reuters news agency reported.