China released a sweeping blueprint today to tighten its control over Hong Kong, revealing plans to establish a security agency in the territory to help Beijing extinguish challenges to its power after months of unrest.

The planned national security law for Hong Kong also gives the territory’s chief official, who must answer to Beijing, the power to appoint which judges will hear such security cases, eroding the autonomy of the city’s independent judiciary.

The announcement drew immediate protests from opposition leaders who warned that it would imperil the rule of law in Hong Kong, a global financial center with greater freedoms than in mainland China.

The proposed law is a pillar of President Xi Jinping’s push to subdue political strife in Hong Kong, the sole part of China that has loudly defied his drive to entrench authoritarian control. Opposition from Western countries appears unlikely to derail that effort.

The details released by the official Xinhua News Agency on Saturday suggested that the law would greatly magnify the Chinese government’s ability to extinguish political opposition in Hong Kong, which sustained monthslong street protests there last year that often flared into clashes with the police. The law would also allow Beijing to override the city’s local laws.

“This is a dramatic change in the administration of justice in Hong Kong, and it gives central authorities control over Hong Kong that was never anticipated” when Britain returned the territory to Chinese sovereignty in 1997, said Jerome Cohen, a New York University law professor.

Defying some expectations in Hong Kong, Chinese lawmakers — meeting as the National People’s Congress Standing Committee — did not vote to approve the law Saturday. Even so, Chinese news media and law experts have said that the government is eager to bring the law into force quickly.

Hong Kong’s opposition politicians said the law would seriously erode the city’s cherished judicial independence and rights to protest and free speech. Chinese security forces already operate secretively in Hong Kong, but the new law would expand and formalize their presence.

“This will hollow out Hong Kong, as far as I could see. This new law can simply mean anything Beijing wants it to mean,” said Claudia Mo, a pro-democracy lawmaker in the Hong Kong legislature. Beijing “used to be quite insistent about judicial independence in Hong Kong,” she said. “Now they are taking off even that facade.”

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