Seven Hong Kong pro-democracy politicians have either been detained or faced arrest on Saturday, with a police statement saying one of them had been charged.
The move was expected to escalate public fury during months of unrest.
Politicians told a news conference their colleagues are expected to be charged with obstructing the local assembly during a raucous meeting on May 11 over a now-shelved China extradition bill that sparked five months of protests.
All of those detained or arrested were expected to face court on Monday.
The weekend development comes after a student who fell from a parking garage when police fired tear gas during clashes with anti-government protesters died on Friday, fuelling more outrage against authorities in the semi-autonomous Chinese territory.