China has warned the UK not to interfere with Hong Kong following the imposition by Beijing of a sweeping new national security law.
Ambassador Liu Xiaoming said the UK's offer of a path to citizenship for up to three million Hong Kongers amounted to "gross interference".
British Foreign Secretary Dominic Raab rejected the accusation.
Opponents of the new law say it erodes Hong Kong's freedoms as a semi-autonomous region.
Earlier, the Hong Kong pro-democracy activist Joshua Wong issued a plea for the world to stand in solidarity with the territory.
Ambassador Liu said he hoped the UK would reconsider its citizenship offer.
"The UK government keeps making irresponsible remarks on Hong Kong affairs," he told a virtual news conference.
The ambassador said a decision on exactly how Beijing intended to respond would be made once it knew the details.
Mr Liu also warned the UK that if it decided against using Chinese tech giant Huawei's technology to build its 5G network, this would send a "very bad message to other Chinese businesses".
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.
Foreign Secretary Dominic Raab insisted the plan to allow millions of British Nationals (Overseas) to come to the UK did not represent interference in China's domestic affairs.
"We want a positive relationship with China... but the real issue here is one of trust, and whether China can be trusted to live up to its international obligations and its international responsibilities," he said.
"That's the message that we are telegraphing along with many of our allies, and indeed many international partners around the world."
Earlier, a spokesman for Prime Minister Boris Johnson urged China not to interfere if Hong Kongers sought to come to the UK.
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.
'Barely veiled warning'
By Paul Adams, BBC diplomatic correspondent
Ambassador Liu is never anything less than robust. His condemnations of what Beijing regards as British interference in China's internal affairs are familiar.
But with Britain and China now at loggerheads over at least two major issues - Hong Kong and Huawei - the sense of a fraying relationship is stronger than ever.
All efforts to thwart the will of 1.4 billion Chinese people, Mr Liu warned, were doomed to failure. When the principles of sovereign equality and non-interference were violated, he said, then relationships would inevitably suffer "setbacks, even retrogression".
And when it comes to Huawei, Boris Johnson's suggestion last week that China might now be seen as a "hostile state vendor" is clearly taken as a serious affront. Chinese officials, the ambassador said, had never described Britain this way.
The ambassador did not spell out in any detail the consequences of Britain changing course on Huawei, or of Britain's offer of citizenship to British National (Overseas) passport holders.
But his use of a quote he attributed to former US National Security Adviser Zbigniew Brzezinski - "If we make China an enemy, China will become an enemy" - was a barely veiled warning.
Numerous other countries, including the US, Canada, Japan and Australia, have also expressed concern over the imposition of the law.
The new law, which was brought in last week, targets secession, subversion and terrorism with punishments of up to life in prison.
Opponents like Mr Wong say it effectively ends freedom of speech. Beijing rejects this.
Mr Wong, who appeared in court on Monday with two other activists on charges related to last year's civil unrest in the territory, said the law was already having a chilling effect.
Education officials in Hong Kong have reportedly ordered the removal from schools of all books that violate the law.
According to a statement sent to Reuters news agency, officials say schools should not provide such reading material unless it is used it to "positively teach" students about the issue.
Books by Mr Wong and other pro-democracy activists were removed from public libraries over the weekend.
Mr Wong said he was determined to keep fighting such moves.
"We know now it's an uphill battle, but no matter we have our friends in the global community continue their international advocacy," he told reporters outside court.
"In Hong Kong, we still urge people to vote in the upcoming primary election scheduled on this weekend.
"We also encourage more people in Hong Kong or in the global community to continue to let Beijing aware [sic] that to kowtow to China is not an option and we must stand up and fight."
At the court hearing, Mr Wong and fellow activist Ivan Lam pleaded not guilty to charges of inciting, organising and taking part in an unlawful assembly in June 2019. Another activist, Agnes Chow, pleaded guilty to two charges.
The first person charged under the new security law was denied bail at another hearing on Monday. Tong Ying-kit is accused of driving a motorbike into police while carrying a sign saying "Liberate Hong Kong" at a demonstration last Wednesday.
What is the security law?
The law is wide-ranging, and gives Beijing powers that it has never had before to shape life in Hong Kong. The law makes it an offence to incite hatred of China's central government and Hong Kong's regional government.
It also allows for closed-door trials, wire-tapping of suspects and the potential for suspects to be tried on the Chinese mainland.
Acts including damaging public transport facilities - which often happened during the 2019 protests - can be considered terrorism.
During an investigation into a possible offence of endangering national security, a senior police commander may authorise officers to enter properties without a warrant to search for evidence "in urgent situations", the gazette says.
Police officers may also ask a magistrate to require a suspect to surrender their travel document in order to stop them from leaving Hong Kong.
The secretary for security may freeze someone's property if they have "reasonable grounds" to suspect it is related to a national security offence.