In 2020, Hong Kong was transformed when China introduced the controversial national security law (NSL).
Created in response to the protests which erupted in Hong Kong the year before, the law made illegal a wider range of dissenting acts while reducing the city's autonomy in several ways.
Beijing and Hong Kong authorities argue the law is necessary to maintain stability and deny it has weakened autonomy.
But critics have called it "the end of Hong Kong" and say it has created a climate of fear in the city.
Among other things, Hong Kong's NSL criminalises anything considered as secession, which is breaking away from China; subversion, which is undermining the power or authority of the central government; terrorism, which is using violence or intimidation against people; and collusion with foreign or external forces.
These four main crimes are punishable with a maximum sentence of life in prison.
Other notable provisions include:
The city has seen the arrests of a number of protesters, activists and former opposition lawmakers.
In July 2023, Hong Kong authorities revealed that about 260 people have been arrested and 79 of them had been charged.
In what is the largest national security case to date, a group known as the Hong Kong 47 have gone on trial for "subversion", for their involvement in organising an unofficial legislative council election primary.
They include some of the city's more prominent pro-democracy figures such as media tycoon Jimmy Lai and activist Joshua Wong.
Numerous pro-democracy news outlets in Hong Kong have shut down, including Lai's Apple Daily, which was known to be critical of the mainland Chinese leadership.
"The arrests worsened the feeling of fear prevalent among journalists since the enactment of the national security law," Chris Yeung, a veteran journalist and former chairman of the Hong Kong Journalists' Association, had earlier told the BBC.
He said the law had "caused far more damaging impacts on freedoms and way of life" than many people had previously envisaged.
The NSL is considered to be a major reason for tens of thousands of Hongkongers choosing to leave the city in recent years. Many have ended up in the UK after the British government introduced a special visa.
But even those who have left have not been spared the reach of the NSL. In recent months, Hong Kong police has issued NSL arrest warrants and offered bounties for several people who are now overseas, such as activist Nathan Law.
Hong Kong was handed back to China from British control in 1997, but under a unique agreement – a mini-constitution called the Basic Law and the "one country, two systems" principle which protects the city's autonomy.
The Basic Law also protects certain freedoms for Hong Kong: freedom of assembly and speech, an independent judiciary and some democratic rights – freedoms that no other part of mainland China has.
Under this same agreement, Hong Kong had to enact its own national security law – this was set out in Article 23 of the Basic Law. It had attempted to do this once before in 2003, but lawmakers withdrew their support amid widespread public protests.
But in 2019, China proposed an extradition bill that could potentially allow Hong Kong suspects to be sent for trial in China.
Anger over the potential bill erupted into some of the largest protests Hong Kong had ever seen, turning into a broader anti-China and pro-democracy movement.
In response, China passed the NSL to ensure the city had a legal framework to deal with what it saw as challenges to Hong Kong's security and Beijing's authority.
It did this by introducing the NSL essentially by decree and bypassing the city's parliament. Critics say this breached the"one country, two systems" principle.
UK Foreign Secretary David Cameron has also said the NSL is "a clear breach" of the agreement the UK struck with China for the handover.
He said the law's "continued existence and use is a demonstration of China breaking its international commitments.
"It has damaged Hong Kong, with rights and freedoms significantly eroded. Arrests under the law have silenced opposition voices," he said.
Beijing and Hong Kong have argued the NSL ensures the "resolute, full and faithful implementation" of "one country, two systems".
If you want to find out more about China-Hong Kong tensions read here:
UPDATE: This article was updated to clarify details about Hong Kong's national security law.
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