Anyone in Hong Kong found guilty of secession, subversion or terrorism will now be sentenced to life in prison under the controversial new security law passed by China this week.

The details emerging about the law, which went into effect Tuesday, come amid fresh concerns in Hong Kong and abroad that it will be used to curb opposition voices in the Asian financial hub.

“The speed and secrecy with which China has pushed through this legislation intensifies the fear that Beijing has calculatingly created a weapon of repression to be used against government critics, including people who are merely expressing their views or protesting peacefully,” said Joshua Rosenzweig, head of Amnesty International’s China Team.

Protesters gather at a shopping mall during a pro-democracy protest against Beijing's national security law in Hong Kong on Tuesday. (AP)

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He called the law’s passage “the greatest threat to human rights in the city’s recent history.”

The legislation will be enforced by a national security committee that is being established under Beijing’s control, according to Sky News. The station says the group’s operations will be kept under wraps and its decisions cannot be challenged by courts in Hong Kong.

The list of offenses that could lead someone to be charged with a serious crime are wide-ranging, as the terrorism charge includes disrupting public transport, while the subversion one includes preventing Chinese or Hong Kong government agencies from performing their duties, Sky News reports.

And the charges can be imposed on people who aren’t permanent residents of the semi-autonomous territory as well, Sky News also reported, citing state media.

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Pro-China supporters holding Chinese national flags, toast during a rally to celebrate the approval of a national security law for Hong Kong, in Hong Kong, on Tuesday. (AP)

"It marks the end of Hong Kong that the world knew before,” activist Joshua Wong reportedly wrote on Twitter after the law went into effect.

Wong, along with fellow activists Agnes Chow and Nathan Law, issued statements on Facebook saying they would withdraw from their organization Demosisto, which then announced that it would disband with the loss of its top members.

Wong said “worrying about life and safety” has become a real issue and nobody will be able to predict the repercussions of the law, whether it is being extradited to China or facing long jail terms.

Ahead of the law’s passage, the Trump administration said Monday it will bar defense exports to Hong Kong and will soon require licenses for the sale of items that have both civilian and military uses.

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“We cannot risk these items falling into the hands of the People’s Liberation Army, whose primary purpose is to uphold the dictatorship of the (ruling Communist Party) by any means necessary,” Secretary of State Mike Pompeo said in a statement.

On Wednesday, Hong Kong's government will mark the 23rd anniversary of the territory's passing from British to Chinese control. A series of official events are scheduled and a heavy police presence is expected to deter any anti-government protests of the type that rocked the city for the second half of last year.

The Associated Press contributed to this report.