Cardinal Joseph Zen has said he is ready to face arrest and trial under Hong Kong's new national security law.

The 88-year-old said in a video posted to his Facebook page that Hong Kong must "be prepared for the unthinkable" after Beijing imposed sweeping new laws cracking down on free speech and dissent in the region. 

"I shall be prudent; I do not seek to offend, but when I deem it necessary, I will say it," said the cardinal, who is an outspoken critic of China.

"If such right and proper words are considered to be against their law, I will endure all the suing, trials and arrests. Numerous predecessors have endured similarly.  We have seen how God has always helped them."

The national security law came into force on July 1, the 23rd anniversary of the handover of Hong Kong from Britain in 1997.

Under the law, supposed acts of subversion, secession, terrorism, or foreign interference can now be tried by a special agency set up by Beijing. 

The cardinal continued: "To implement this national security law, it's not very sensible of them [the Chinese Communist Party, CCP].

"They won't benefit from destroying Hong Kong.

"Perhaps they are truly insane. Who knows? Let them be then. Isn't there a saying, 'Those whom God wishes to destroy, he first makes mad'?"

Hours after the law's passing, Christian pro-democracy activist Joshua Wong stepped down from his role in the campaign group Demosisto citing fears that he could be extradited to China to face a 10-year "political imprisonment".

Prime Minister Boris Johnson has called the passing of the law a "clear and serious breach" of the 1985 Sino-British joint declaration protecting certain freedoms for the first 50 years after the handover.

In the Commons on Tuesday, he repeated Britain's offer of citizenship for Hong Kongers. 

"It violates Hong Kong's high degree of autonomy and threatens the freedoms and rights protected by the joint declaration," he said.

"We made clear that if China continued down this path we would introduce a new route for those with British National (Overseas) status to enter the UK, granting them limited leave to remain with the ability to live and work in the UK and thereafter to apply for citizenship.

"And that is precisely what we will do now."

International Christian Concern has said it fears for the fate of clergy and Christians under the new law. 

Gina Goh, ICC's Regional Manager for Southeast Asia, condemned the imposition of the law, saying that it "strips away the autonomy and freedoms of Hong Kong".

"Xi Jinping's regime has done nothing but erode Hong Kong's human rights and justice since he took office in 2012," she said.

"It is time now for the international community to stand in solidarity with Hong Kong and take necessary measures to punish the CCP."