Critics contend that the law would allow virtually anyone in the city to be picked up and detained in mainland China, a country in which judges must follow the orders of the Communist Party. They fear the new law would target not just criminals but political activists as well.

The extradition plan applies to 37 crimes. That excludes political ones, but critics fear the legislation would essentially legalize the sort of abductions to the mainland that have taken place in Hong Kong in recent years. (The mainland authorities are typically not permitted to operate here.)

Under the law, the chief executive would need to approve an extradition request before an arrest warrant is issued. A Hong Kong court would also be empowered to check that there is a basic case against a suspect.

Yet Hong Kong’s subordinate status to the mainland would make it extremely difficult for a local leader to reject an extradition request from her superiors.

Taiwan, a self-governing democracy, has said it will not comply with any extradition agreement that defines it as a part of China. And many in Hong Kong — where the government has ousted opposition lawmakers and rejected demands for free elections — see the extradition plan as the endgame of a long battle to disable dissent and political opposition in their city.