The Hong Kong police have rounded up a number of prominent activists and three lawmakers for their roles in protests since June 9.
The move came ahead of news that the authorities have upheld a police ban on a proposed rally and march today.
The arrests included Demosisto party's secretary-general Joshua Wong and member Agnes Chow, and coincided with claims of attacks on activists, including mass rally organiser Jimmy Sham, by unknown men.
Both Wong and Chow, arrested yesterday morning, are accused of taking part in a June 21 unauthorised demonstration outside Wan Chai police station and inciting others to join it. Wong, a founder of the pro-self-determination party, faces a further charge of organising the illegal demonstration. The two, now out on bail of HK$10,000 (S$1,800) each and under curfew, will have their cases heard on Nov 8.
Wong was released from prison in mid-June after he served a two-month sentence for his role in the 2014 Umbrella Movement protest.
Another Demosisto member Ivan Lam, who left the city on Wednesday, faces a count of incitement.
Others detained included former University of Hong Kong student union president Althea Suen and Sha Tin district councillor Rick Hui. Civic Passion leader and lawmaker Cheng Chung Tai, Civic Party's Jeremy Tai and independent Au Nok Hin were also arrested.
On Thursday night, the leader of the banned Hong Kong National Party, Andy Chan Ho Tin, was detained at the airport while preparing to board a flight. He was among eight people arrested for possession of offensive weapons following a raid in Sha Tin earlier this month, where officers found materials for making petrol bombs and other weapons.
There were also claims of violence against activists. On Thursday, Mr Sham, convener of Civil Human Rights Front, and Mr Max Chung, organiser of a July anti-triad protest in Yuen Long, were attacked hours apart by masked men.
Mr Chung and the reporter he was with sustained injuries, while Mr Sham emerged unscathed but his friend was hit.
The developments came ahead of news that the authorities have upheld a police ban on the Civil Human Rights Front's application for a rally and a march today.
The group has organised the biggest street marches in Hong Kong since the handover in 1997, including the one on June 16 with two million people and the one on Aug 18 with a turnout of 1.7 million.
Its spokesman Bonnie Leung called the rejection of its appeal against the police ban "a total violation" of human rights, saying people would still take to the streets today.
The march is to mark the fifth anniversary of Beijing's announcement of a political reform framework which stated that there must be screening for the Chief Executive election in the city, but this was eventually rejected by the Legislative Council. The move resulted in the Umbrella Movement that lasted 79 days, during which key roads in the city centre were occupied.
There are also calls online for people to join a religious assembly in Wan Chai today, while a two-day strike from next Monday has been planned. The strike marks the second such call after Aug 5, when protests were held in multiple districts and later turned violent and chaotic.
Hong Kong's flagship carrier Cathay Pacific has warned staff that they risk getting fired if they join the strike, Bloomberg reported.
A China Daily editorial yesterday said Chinese soldiers stationed in Hong Kong have "no reason to sit on their hands" if the situation worsens.
The protests began after the government mooted a controversial Bill - now suspended - that would allow the authorities to extradite people to jurisdictions with which Hong Kong has no formal extradition agreements, including mainland China. But the protests have since morphed into a broader movement seeking universal suffrage and an independent probe into alleged police brutality.