National Security Law:
More than 50 of Hong Kong's most prominent pro-democracy activists and politicians have been arrested in the biggest crackdown since China imposed a draconian security law last year.
About 1,000 police took part in morning raids on 72 premises across the city.
Those held helped run an unofficial "primary" to pick opposition candidates ahead of postponed 2020 elections.
They are accused of trying to "overthrow" the government. Activists say the new law aims to quash dissent.
China's government imposed the legislation on the semi-autonomous territory in June, saying it was necessary to curb months of sometimes violent pro-democracy protests.
Beijing defended Wednesday's arrests, with Foreign Ministry spokeswoman Hua Chunying saying they were needed to stop "external forces and individuals [colluding] to undermine China's stability and security".
But the crackdown appeared to confirm the fears of many who warned about the reach of the law, with Amnesty International saying the arrests are "the starkest demonstration yet of how the national security law has been weaponised to punish anyone who dares to challenge the establishment".
Who was detained?
Dozens have been arrested under the security law since it was enacted in June, but never have so many been rounded up at the same time as on Wednesday, which marked a major escalation in the crackdown and sent a chill through pro-democracy figures.
Police fanned out around the city and in early morning raids detained 53 people who represent a who's who of democratic forces in the former British colony, young and old. They include:
long-established opposition figures James To, Lam Cheuk-ting, Claudio Mo and Benny Tai, one of the initiators of the primaries
young pro-democracy campaigners Lester Shum, Gwyneth Ho, Tiffany Yuen and Jeffrey Andrews
Human rights lawyer John Clancey, a US citizen
two academics behind the primaries strategy
13 candidates from the primaries
Police also searched the home of already detained activist Joshua Wong, as well as three news outlets.
Apple Daily and Stand News, which had co-organised election forums for the primaries candidates, and In-Media, which ran ads around the primaries, were ordered to hand over information.
Mr Clancy, a veteran legal activist and long-time resident of Hong Kong, is chairman of the Asian Human Rights Commission and the first US citizen held under the new law. He was detained when police searched the offices of law firm Ho Tse Wai & Partners where he works.
Hours after the dawn arrests John Lee, Hong Kong's security secretary, confirmed a group of people had been arrested as part of an operation targeting those suspected of "overthrowing" the city's government.
The government will not tolerate "subversive" acts, he said, speaking at the city's Legislative Council.
If charged under the new national security law, the detainees could face life in prison.
The BBC's Danny Vincent in Hong Kong says the legislation has spread what activists call "white terror" across the city. It has all but silenced the street protest movement and led to a growing number of activists fleeing the territory.
When authorities introduced the law, they claimed it would target only a small number of activists, but the wide-ranging nature of Wednesday's operation has led many to fear the authorities are now trying to eliminate the entire opposition camp, he says.