Western leaders, alongside the United Nations and human rights groups, have raised their voices in disapproval of Hong Kong's latest security law, arguing that it poses a significant threat to the already diminishing freedoms in the region.

Hong Kong's pro-Beijing legislature unanimously passed Article 23, as it's known locally, targeting a range of offenses considered treasonous, reported the BBC.

Officials have said the law is essential for stability, while opponents called it a "nail in the city's coffin."

China has long advocated for the law and emphasized "smears" by critics would fail.

Under the new legislation, closed-door trials are authorized, the police are given the right to detain suspects for up to 16 days without charge, and penalties, including life sentences, are outlined, among other elements.

"The new national security legislation is going to double down the repression on freedoms in Hong Kong with extended egregious sentences and a broadened definition of national security," said Frances Hui, an activist in the US.

In a joint statement released on Tuesday, 81 lawmakers and public figures from across the globe, including the UK, US, Canada, and South Korea, expressed "grave concerns" over the legislation.

The law, which expands upon Beijing's 2020 National Security Law, now criminalizes secession, subversion, terrorism, and collusion with foreign forces.

"The legislation undermines due process and fair trial rights and violates Hong Kong's obligations under international human rights law, jeopardizing Hong Kong's role as an open international city," the statement said, calling it another "devastating blow" to freedom.

Alarmed by the "sweeping and vaguely defined" provisions in the legislation, the United States' sentiment was also shared by the European Union, which expressed concerns about the potential impact of the law on the city's business center status.

UK Foreign Secretary David Cameron condemned the law, warning that it would "further diminish rights and freedoms" and "entrench a culture of self-censorship" in the former British colony.

Hongkongers shared their concerns with the BBC, admitting they are already censoring themselves in conversations with friends and colleagues as they fear the rise of an "informant culture" in the city.

The Chinese Embassy in the UK responded to Cameron's remarks, calling them rubbish and "a serious distortion of the facts."

China's government pushed back against criticisms of Article 23, saying it is "unswervingly determined to safeguard national sovereignty, security and development interests, implement the 'one country, two systems' policy, and oppose any external interference in Hong Kong affairs."

"All attacks and smears will never succeed and are doomed to fail," foreign ministry spokesman Lin Jian told a regular press conference in Beijing.

Hong Kong's leader, John Lee, defended the law, which accelerated through its final phase on Tuesday.

Lee asserted that the legislation would assist the city in "effectively preventing, suppressing, and penalizing espionage activities, conspiracies, and schemes from foreign intelligence agencies, as well as infiltration and sabotage by hostile forces."

"From now on, the people of Hong Kong will no longer experience these harms and sorrows," he added.

However, those who championed the pro-democracy protests against China's expanding control over the city see the new law as another loss.

It brings Hong Kong "one step closer to the system of mainland China", former Hong Kong lawmaker Nathan Law, who is now in exile in the UK, told the BBC's Newsday program.

"The chilling effect... and the result of a collapse of civil society is impacting most Hong Kong people."

Hong Kong's last British governor, Chris Patten, called the legislation "another large nail in the coffin of human rights and the rule of law in Hong Kong and a further disgraceful breach of the Joint Declaration."

The UK transferred Hong Kong to China in 1997 under the principle of "one country, two systems," which promised the city a certain level of autonomy.

Despite assurances from Beijing and Hong Kong, critics and international rights groups argue that China's control over the city has only strengthened over time.