China promised Thursday to take countermeasures against Britain if it presses ahead with plans to extend citizenship rights to Hong Kongers after Beijing imposed a sweeping security law on the restless financial hub.

Beijing has faced a groundswell of criticism from primarily Western nations over its decision to impose a new law outlawing acts of subversion, secession, terrorism and colluding with foreign forces.

Adding to concerns, Hong Kong's influential Bar Association published a new legal analysis warning that the wording of the law-which was kept secret until Tuesday-undermines the city's independent judiciary and stifles freedoms.

Britain has said the law breaches China's pre-handover "One Country, Two Systems" promise to grant residents key liberties-as well as judicial and legislative autonomy-until 2047.

It has responded by announcing plans to allow millions of Hong Kongers with British National Overseas status to relocate with their families and eventually apply for citizenship.

"We will live up to our promises to them," foreign secretary Dominic Raab told parliament.

That move has infuriated Beijing, which says Britain promised not to grant full citizenship rights to Hong Kongers ahead of the 1997 handover.

"If the British side makes unilateral changes to the relevant practice, it will breach its own position and pledges as well as international law and basic norms governing international relations," China's embassy in London said Thursday.

"We firmly oppose this and reserve the right to take corresponding measures," it added. Britain is not alone in announcing plans to offer Hong Kongers sanctuary or increased immigration rights as fears multiply over the semi-autonomous city's future under the new law.

On Thursday, Australian leader Scott Morrison said he was "very actively" considering offering Hong Kongers safe haven.

Reuters from Tripel adds: Taiwan citizens should avoid unnecessary visits to or transit through Hong Kong, Macau or mainland China after the passing of an "outrageous" national security law for the former British colony, a Taiwan government official said on Thursday.

The law, imposed on Hong Kong by Beijing, targets crimes like secession, subversion, terrorism and collusion with foreign forces with up to life in prison. It has been widely condemned in Chinese-claimed and democratic Taiwan where Hong Kong's anti-government protesters have won support.

Speaking to reporters in Taipei, Chiu Chui-Cheng, deputy head of Taiwan's Mainland Affairs Council, said the law was the "the most outrageous in history" with a reach that extended everywhere.

"Taiwan people should avoid making unnecessary visits to or transits via Hong Kong, Macau and the mainland," he said. Beijing denies stifling Hong Kong's freedoms.

Chiu said Taiwan's de facto consulate in Hong Kong - the island has no formal diplomatic relations with China - would continue to operate.

"We won't take the initiative to withdraw it, unless there are external factors," he said. "We will stay until the last minute."

Hong Kong has traditionally played an important role in China-Taiwan interactions, especially when there were no regular direct flights before 2008, and its airport is often used by Taiwan people making flight connections.