China has warned the UK not to interfere with Hong Kong following the imposition of a new national security law, as one pro-democracy campaigner begged for international support.

Ambassador Liu Xiaoming said the UK's offer of a path to citizenship for up to three million Hong Kongers amounted to "gross interference".

The offer came after Beijing brought in the controversial and sweeping new law.

Opponents say it erodes the territory's freedoms as a semi-autonomous region.

Activist Joshua Wong had earlier called for more support, asking his fellow Hong Kongers and the wider world not to "kowtow" to Beijing.

But Ambassador Liu said he hoped the UK would reconsider its offer.

"The UK government keeps making irresponsible remarks on Hong Kong affairs," he told reporters.

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The ambassador said a decision on exactly how Beijing intended to respond to the offer would be made once it knew the details.

The UK has argued that China has reneged on an agreement that took effect in 1997, which offered certain freedoms to Hong Kong for 50 years in return for handing the territory back to Beijing.

Later on Monday, a spokesman for UK Prime Minister Boris Johnson urged China not to interfere if Hong Kongers with British National (Overseas) status sought to come to the UK.

"We would expect China to understand the importance of adhering to international law," the spokesman said.

He added: "We are currently assessing the national security law and its legal ramifications in terms of extradition with Hong Kong.

"There are already extensive extradition safeguards in the UK. The courts are required to bar a person's extradition to any country if it would be incompatible with their human rights or if the request appears to be motivated by their political opinion."

Also on Monday, Facebook and its messaging service WhatsApp said they had "paused" processing requests for information from the Hong Kong government and law enforcement agencies "pending further assessment of the impact of the national security law".

The assessment will include "formal human rights due diligence and consultations with human rights experts", according to a statement.

BBC