Hong Kong offered a bounty for the arrest of a former British consulate worker who fled after being tortured.
Authorities offered to pay HK$1,000,000 (£100,400) for tips to arrest Simon Cheng, who was granted asylum in Britain after a high-profile case in 2019.
The move was seen as a threat to British sovereignty and jurisdiction, sparking angry exchanges between London and Beijing on Friday.
Britain called Hong Kong's action a "threat to our democracy and fundamental human rights," and Lord Cameron, the foreign secretary, said he had instructed British officials in Hong Kong, Beijing and London to "take up this matter urgently." to mention".
China hit back, saying the bounty was "necessary and legitimate."
"By encouraging these anti-China individuals who are wreaking havoc in Hong Kong, the United States and Britain are exposing their evil intentions to ruin Hong Kong," said Mao Ning, a spokesman for China's Foreign Ministry.
Mr Cheng was one of five foreign activists for whom Hong Kong has placed a bounty, with authorities vowing to pursue them "until the end".
He and Francis Hui, Joey Siu, Johnny Fok and Tony Choi all fled Hong Kong after Beijing imposed a draconian national security law that introduced harsh penalties for vaguely defined crimes in an effort to suppress the city's pro-democracy movement.
Police Chief Inspector Steve Li Kwai-wah said Thursday that the five activists are suspected of incitement to secession, incitement to subversion and foreign conspiracy - crimes that carry a prison sentence of up to life in prison.
"All of them, who have already fled abroad, have continued to commit crimes under the national security law that seriously endanger national security," Li said.
Mr Cheng responded to the allegations on social media. He said: "Being hunted by the Chinese secret police, among a million[HK] dollar premium, is a lifelong honor. If the government considers the quest for democracy and freedom a crime, we embrace the accusations to reveal the real face of social justice, unyielding to authority."
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Ms. Siu, a US citizen, pointed out that she was persecuted for exercising freedoms in her own country. "I will never be silenced, I will never back down," she said.
Washington echoed Britain's complaints. It said it deplored any attempt to apply the national security law extraterritorially and that "advocates of democracy and freedom will continue to enjoy their constitutionally guaranteed freedoms" in the US.
China is known to pursue dissidents using a network of overseas police stations, quasi-legal extensions of the Communist Party state accused of meddling in British politics.
Amnesty International said the decision was further confirmation that the Hong Kong authorities' systematic dismantling of human rights has officially become global.
"The brazen tactic of placing Wild West-style bounties on the heads of activists appears to be becoming a method of choice to silence dissent," said Sarah Brooks, deputy regional director for China.
Amnesty and other rights groups have also condemned the national security trial of Jimmy Lai, the Hong Kong pro-democracy activist and founder of the now-defunct Apple Daily newspaper, which begins on Monday.
"This case has been an attack on freedom of the press and freedom of speech from the very beginning," Ms. Brooks said. "The Hong Kong authorities must immediately and unconditionally release Jimmy Lai and expunge his criminal convictions."
Hong Kong's controversial national security law has reshaped society in the former British colony and breached the legal firewall that once existed between the city and mainland China.
It claims the power to hold accused people around the world accountable, although Hong Kong authorities have not specified how enforcement abroad might be possible.
The bounties are the second set of hefty rewards awarded by Hong Kong police in their pursuit of fugitives accused of national security crimes.
Eight prominent activists from abroad - including pro-democracy lawmakers Nathan Law and Ted Hui - were identified in July as targets for police, who offered a HK$1 million bounty each for information leading to their arrest.
Lai's son said Friday that he thought the outcome of his father's trial had already been determined, but that he was proud of him for standing up for his beliefs.
Life imprisonment
Mr Lai faces life in prison on charges of collusion with foreign forces, including the US. He is one of Hong Kong's most prominent critics of the Chinese Communist Party's leadership and has faced a volley of lawsuits since a wave of pro-democracy demonstrations in 2019.
He is already serving a prison sentence of five years and nine months for a fraud conviction related to a lease dispute for his newspaper. Mr Lai has pleaded not guilty to all charges he faces in his retrial.
Sebastien Lai, one of his sons, said the trial, with three government-appointed judges and no jury, was a sham.
"There's actually no fear involved when you think about it because it's a complete show process. The result is already determined in advance," says Sebastien, who lives in Taiwan.
The Hong Kong government did not immediately respond to a request for comment. Hong Kong and Chinese authorities say the city's rule of law is robust and everyone there is treated equally. Both Hong Kong and Chinese officials said the security legislation was needed to restore stability in the former British colony.
Sébastien said he understood that his father was "doing well."
"He is 76 years old and has been in solitary confinement for three years, so I can't imagine what that does to someone physically and mentally," he added.
He hasn't seen his father in three years and said he misses simple things like family dinners.
"I always go back and forth - would I rather have my father with me instead of standing up for the freedom of others? And my conclusion is that I am very inspired by him and I am very proud that he is my father," he said.
