Hong Kong police arrested 53 people in dawn raids on democracy activists on Wednesday in the biggest crackdown since China last year imposed a new security law which opponents say is aimed at quashing dissent in the former British colony.

Hong Kong’s most prominent pro-democracy advocates were arrested in raids on 72 premises as the authorities said last year’s unofficial vote to choose Opposition candidates in city elections was part of a plan to “overthrow” the government.

The arrests were linked to an unprecedented, independently organised and non-binding vote to select Opposition candidates for a since-postponed legislative election.

About 1,000 police took part in the raids, which included searches of the offices of a pollster and a law firm.

“The Chinese Communist Party has further turned the screw in Hong Kong,” said Chris Patten, the last British governor of Hong Kong. “Liberal democracies around the world must continue to speak out against the brutal destruction of a free society.”