Protesters have forced their way into Hong Kong’s legislature, and are engaged in a standoff with police inside.
Dozens of demonstrators broke through the glass of the Legislative Council (LegCo) building, while a large crowd observed the violence from outside.
Police held signs warning they would use force if protesters charged, but have yet to move against the crowd.
The violence is a breakaway part of a peaceful protest involving thousands.
Pro-democracy demonstrators had taken to the streets on the anniversary of the city’s handover from UK to Chinese rule.
Police used pepper spray and batons to contain crowds during earlier clashes.
This is the latest in a series of protests against a controversial bill that would allow extraditions to mainland China.
The government has agreed to suspend it indefinitely, but rallies continue amid calls for Chief Executive Carrie Lam to resign.
Hong Kong, a former British colony, has been part of China since 1997 under a “one country, two systems” deal that guarantees it a level of autonomy. Pro-democracy events are held every year to mark the handover.
The LegCo building was put on red alert for the first ever time on Monday – meaning people should evacuate the building and area.
But by 20:00 (12:00 GMT), the crowd had grown rather than dispersed, and protesters seemed able to walk freely in and out of the broken glass at the front of the building.
What happened on Monday?
In the morning, a flag-raising ceremony to mark the handover took place inside the Hong Kong Convention and Exhibition Centre, amid a heavy police presence.
Demonstrators blocked several roads nearby early using items like metal and plastic barriers.
Police officers equipped with shields, batons and pepper spray clashed with hundreds of protesters about 30 minutes before the ceremony.
At least one woman was seen bleeding from a head wound after the clashes, AFP news agency says.
A police statement condemned “illegal acts” by protesters who, it said, had taken iron poles and guard rails from nearby building sites.
Thirteen police officers were taken to hospital after protesters threw an “unknown liquid” at them, police said. Some are reported to have suffered breathing difficulties as a result.
Thousands joined a mostly peaceful pro-democracy march on Monday afternoon.
At about lunchtime, a breakaway group of protesters moved to LegCo where the government meets. The small group began ramming the glass doors with a metal trolley, succeeding in smashing in the door, before largely dispersing.