British Prime Minister Boris Johnson said a contentious anti-subversion law China imposed in the former British colony has brought a chilling effect on Hong Kong's rights and freedoms, as a Hong Kong activist arrested over speech crime was denied bail by a Hong Kong court on Thursday.

"We believe that the security measures that have been brought in by Beijing do sadly amount to a breach of the letter and of course of the spirit of the Sino-British (Joint) Declaration," Johnson told a parliamentary committee meeting in London on Wednesday, referring to the law that was imposed in the territory on June 30.

British Prime Minister Boris Johnson leaves 10 Downing Street for his weekly Prime Minister's Questions (PMQs) appointment at the House of Commons in London, England, on September 16, 2020. (NurPhoto/Getty/Kyodo)

The joint declaration signed by both countries formed the basis on which Hong Kong was returned to Chinese rule in 1997 with undertakings made by China that a high degree of autonomy and rights and freedoms would remain for 50 years.

"You are starting to see a chilling of free speech, you are starting to see...the effect of that security rule, the Chinese legislation, already starting to bite on the people of Hong Kong," Johnson said.

Following the law's enactment, Britain has extended an arms embargo applied to China to Hong Kong as well and suspended an extradition agreement with the territory. Also, Britain has said it will open its door to millions of eligible British National (Overseas) passport holders and their dependents in Hong Kong as an escape route, which Johnson called "the right thing to do."

Tam Tak-chi of the radical pro-democracy group People Power, arrested on Sept. 6 by police' national security officers, was facing six counts of uttering seditious words and was denied bail by the High Court on Thursday. He has to remain in custody until the next court day in November.

Although Tam was not charged for breaking the national security law, which targets sedition, subversion, terrorism and collusion with foreign forces, at least 26 others have been arrested for alleged violation of the law since it took effect, including media mogul Jimmy Lai, activists Agnes Chow and Andy Li.

Li and 11 others who were detained in mainland China since August following a reported botched attempt to flee Hong Kong for Taiwan have even been accused of seeking Hong Kong's separation from China by Foreign Ministry spokeswoman Hua Chunying, before any of them was officially charged.

"We urge China to live up to its international obligations and it is absolutely crucial that the freedoms of the people and the autonomy of Hong Kong are respected in full," British Foreign Secretary Dominic Raab told reporters after meeting with U.S. Secretary of State Mike Pompeo in Washington on Wednesday.

"We will be watching very carefully, not just for the enactment but the application of the national security legislation and how that plays out in the weeks and months ahead," Raab said.

Pompeo praised Britain for speaking up for Hong Kong people and offering many of them refuge in facing "the China challenge."

"The United Kingdom and the United States I know will always stand together in the defense of freedom," Pompeo said.

Apart from suspending the extradition treaty with Hong Kong, the United States has also decided to revoke the special trade status it offered Hong Kong and sanctioned 11 local and Chinese officials deemed accountable undermining Hong Kong's autonomy and restricting the freedom of expression or assembly of its citizens.