HONG KONG (BLOOMBERG) - Escalating violence in Hong Kong, including the shooting of an 18-year-old protester, has led the city's leader to invoke a colonial-era law - still on the books - to try to quell months of unrest with a ban on face masks.

The powers the ordinance, similar to martial law, grants go much further, however, from curfews to confiscations to censorship.

The law could be cited to authorise the blocking of messaging apps or websites favoured by protesters or even to disrupt Internet service entirely in the Asian financial hub - a prospect one trade group official called "worse than having the Chinese army come in".

1. WHAT IS THE EMERGENCY LAW?

The Emergency Regulations Ordinance was introduced by the British in 1922. It has not been used in more than half a century, but can still be invoked in case of emergency or public danger.

It affords the Chief Executive (as Hong Kong's post-colonial leaders are called) the power to make "any regulations whatsoever which he may consider desirable in the public interest".

Its provisions include arrests, property seizures, deportation, control of the ports and transportation - and censorship.

2. WHAT'S BEING DONE?

On Oct 4, the city's Executive Council approved a ban on face coverings at protests and public assemblies, effective at midnight, in a move aimed at making it harder for protesters to hide their identity.

Chief Executive Carrie Lam said the council believed it would be "an effective deterrent and help the police in enforcing the law", adding that other countries have similar bans.

Violators could be sentenced to as long as one year in prison.

She also said things were "fluid" and said that if they worsened, "other means to tackle the situation" would be considered.

Earlier, associations representing front-line police officers and inspectors urged the city to impose curfews.

3. DOES THE LAW COVER THE INTERNET?

It would seem so. The law was last invoked during Hong Kong's 1967 riots - long before the Internet.

But the authority granted specifically covers "the control and suppression of publications, writings, maps, plans, photographs, communications and means of communication".

4. HOW COULD IT BE DISRUPTED?

Ms Lam could order private telecommunication companies to cut Internet services delivered through fixed-line and mobile phone networks.

Alternatively, she could compel these providers to take other measures, including slowing Internet speeds, disabling particular mobile phone networks and public Wi-Fi spots, or blocking certain websites and platforms.

5. WHAT ABOUT APPS?

Many of the digital tools that protesters use to organise demonstrations, including the Telegram messaging app and LIHKG discussion forum, use cloud-hosting services.

That means to block one site hosted on, say, Amazon Web Services, the government would have to block the entire AWS domain, which hosts thousands of websites.

Even without such a move, apps are walking a fine line: In October, Apple rejected a Hong Kong app designed to track police activity in real-time.

6. WOULD IT BE JUST HONG KONG?

The ripple effects could extend far and wide.

The city is one of the largest so-called core nodes of Asia's optical fibre network, and has the region's biggest Internet exchange, which means traffic from outside could be slowed as it flows through Hong Kong. (That includes 80 per cent of mainland China's external Web traffic, according to the Hong Kong Internet Service Providers Association.)

Hong Kong also hosts more than 100 data centres operated by local and international companies, according to the ISP trade group.

So it might be hard or impossible for someone in Singapore, say, to access certain data if it were stored on a server in Hong Kong.

7. COULD SOMEONE GET AROUND IT?

Even if the Hong Kong government were able to block certain sites, residents could use services called virtual private networks, or VPNs, which allow people to bypass these blockades by connecting to servers outside the city.

Russia, for instance, has blocked Telegram, but users continue to flock to the messaging service by using VPNs that circumvent banned sites.

8. CAN COURTS STOP IT?

There is nothing overt in the emergency ordinance that says the judicial system can review the Chief Executive's extraordinary actions, but current and past lawmakers say that citizens could try to challenge them in court.

9. WHAT COMPANIES WOULD BE INVOLVED?

Hong Kong has hundreds of Internet service providers, but fewer than a dozen companies that operate mobile networks or provide actual infrastructure, such as the optical fibre that connects homes and businesses to the Internet.

One of the biggest is PCCW, which is controlled by billionaire Richard Li.

Large technology companies, such as Amazon.com and Alphabet's Google, that operate data centres in Hong Kong would be affected.

Many of these data centres power cloud services in the greater Asia-Pacific region, which in turn serve hundreds of thousands of companies around the world.

10. WHAT ABOUT HONG KONG'S HUB STATUS?

Hong Kong hosts hundreds of international companies, many of which came to the city, rather than mainland China, in part because it was easier to do business in a place that does not censor the Internet or block online tools and social networking websites such as Facebook and Twitter.

The "one country, two systems" framework has allowed Hong Kong's Internet to bypass China's so-called Great Firewall.

"If we lose freedom of the Internet, there is no Hong Kong any longer," said Mr Francis Fong, honorary president of the Hong Kong Information Technology Federation, a trade group with more than 100 members, including Microsoft, Cisco Systems, and IBM.

"This would be worse than having the Chinese army come in."

11. WHERE ELSE HAS THIS HAPPENED?

India cut connectivity in the region of Kashmir in August, when Prime Minister Narendra Modi scrapped its autonomy and imposed a military lockdown.

India has had the highest number of Internet shutdowns in the world: 134 times in 2018, according to technology and human rights non-profit Access Now.

Globally, there were 196 documented shutdowns across 25 countries, up from 75 in 2016, it said.