Several protesters have been arrested while trying to run from a Hong Kong university campus surrounded by police.
A group of around 100 people tried to leave Polytechnic University, but were met with tear gas and rubber bullets.
It is the third time protesters have tried to leave, following a violent and fiery overnight stand-off with police.
In the past week, the university campus has turned into a battleground as the long-running Hong Kong protests become more violent.
As protests were raging, Hong Kong's High Court ruled that a ban on protesters wearing face masks was unconstitutional. The colonial-era emergency law was invoked in October, but protesters largely defied the ban.
In Beijing, Chinese foreign ministry spokesman Geng Shuang said at a news conference: "No-one should underestimate China's will to safeguard its sovereignty and Hong Kong's stability."
Hong Kong is a part of China, and these protests are, in part, about the fear that the special freedoms Hong Kong enjoys as a former British colony are being eroded.
At around 13:45 local time (05:45 GMT), around 100 protesters tried to run from the campus, but were met by police trying to arrest them.
Images showed protesters pinned down by officers amid clouds of tear gas.
Police officers had earlier said protesters could leave the campus via another exit - the Cheong Wan Road South Bridge - but urged them to drop their weapons and remove their gas masks.
But pro-democracy lawmaker Ted Hui, who is on the campus, said the bridge had "been sealed off", and it was not possible to leave this way. Earlier, he said there were up to 1,000 protesters on the site.