Hong Kong security law: What is it and is it worrying?

China stunned Hong Kong when it announced it would impose a national security law on the city. Many worry this could spell the end of Hong Kong's unique freedoms. So what do we know, and what do people fear the most?

What is this law all about?

First of all, nothing is set in stone... yet.

What China has submitted is a draft resolution to its rubber stamp parliament. That will be voted on (and almost certainly passed) this week. Only after that, will it be fleshed out into an actual draft law.

So the details are thin but we know this much. A law would make criminal any act of:

• secession - breaking away from the country
• subversion - undermining the power or authority of the central government
• terrorism - using violence or intimidation against people
• activities by foreign forces that interfere in Hong Kong

One part that has got people worried is the suggestion that China could set up its own institutions in Hong Kong responsible for security.

So it could introduce its own law enforcement agencies, alongside the city's own.

Why did China do this?

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 a so-called "one country, two systems" principle.

They are supposed to protect 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 the same agreement, Hong Kong had to enact is own national security law - this was set out in Article 23 of the Basic Law.

But its unpopularity means it has never been done - the government tried in 2003 but had to back down after protests.

Then, last year, protests over an extradition law turned violent and evolved into a broader anti-China and pro-democracy movement.

China doesn't want to see that happen again.

Why are people in Hong Kong afraid?

As the law has not even been drafted yet, it is hard to be concrete, but essentially people in Hong Kong fear the loss of these freedoms.

China expert Willy Lam is concerned the law could see people punished for criticising Beijing - as happens in mainland China.

People believe this will affect free speech and their right to protest. In China, this would be seen as subversion.

Some pro-democracy activists - such as Joshua Wong - have been lobbying foreign governments to help their cause. Such campaigning could be a crime in the future.

Many are also afraid Hong Kong's judicial system will become like China's.

"Almost all trials involving national security are conducted behind closed doors. It [is] never clear what exactly the allegations and the evidence are, and the term national security is so vague that it could cover almost anything," Professor Johannes Chan, a legal scholar at the University of Hong Kong, says.

Finally, people worry that a threat to Hong Kong's liberties could affect its attractiveness as a business and economic powerhouse.

So can China just push this through?

The Basic Law says Chinese laws can't be applied in Hong Kong unless they are listed in a section called Annex III - there are already a few listed there, mostly uncontroversial and around foreign policy.

These laws can be introduced by decree - which means they bypass the city's parliament and Hong Kong's chief executive Carrie Lam has already said she will co-operate.

If you want a deep dive into the tensions between China and Hong Kong read more here:

• What is the Basic Law and how does it work?
• Why are there protests in Hong Kong? All the context you need
• Profile: Carrie Lam, Chief Executive of Hong Kong
• Hong Kong's year in seven intense emotions

Critics say this amounts to a breach of that "one country, two systems" principle, which is so important to Hong Kong.

If there were sanctions associated with any national laws to be included in the annex, Professor Chan also says it should go through Hong Kong's parliament because the judicial systems are so different.

"The values underlying the criminal justice system in two jurisdictions are so different that any criminal law should only be enacted by HK and not by the mainland," he says.

What's more, the draft resolution in itself goes against Article 23, says Professor Chan - because that says Hong Kong must draft its own security law.

So that suggests the Hong Kong government still needs to do that - which could make things complicated.

Reporting by the BBC's Grace Tsoi