Police cited the law in confronting protesters.

“You are displaying flags or banners/chanting slogans/or conducting yourselves with an intent such as secession or subversion, which may constitute offences under the … national security law,” police said in a message displayed on a purple banner.

Officials in Beijing unveiled the details of the much-anticipated law late on Tuesday after weeks of uncertainty, pushing China’s freest city and one of the world’s most glittering financial hubs on to a more authoritarian path.

As thousands of protesters gathered downtown for an annual rally marking the anniversary of the former British colony’s handover to China in 1997, riot police used pepper spray and fired pellets as they made arrests after crowds spilled into the streets chanting “resist till the end” and “Hong Kong independence”.

“I’m scared of going to jail but for justice I have to come out today, I have to stand up,” said one 35-year-old man who gave his name as Seth.

Police said they had made more than 300 arrests for illegal assembly and other offences, with nine involving suspected violations of the new law.

The law punishes crimes of secession, subversion, terrorism and collusion with foreign forces with up to life in prison, and will have mainland security agencies operate in Hong Kong for the first time and allow for extradition to the mainland for trial.

China’s parliament adopted the law in response to protests last year triggered by fears that authorities in Beijing were stifling the city’s freedoms that had been guaranteed by a “one country, two systems” formula agreed when it returned to Chinese rule.

Authorities in Beijing and Hong Kong have repeatedly said the legislation is aimed at a few “troublemakers” and will not affect rights and freedoms, nor investor interests.

But critics fear it is aimed ending the pro-democracy opposition and will crush the freedoms that are seen as key to Hong Kong’s success as a financial centre.

The United States government and some of its allies have criticised the legislation.

The British government said it would stand by its word and offer all those in Hong Kong with British National Overseas status a “bespoke” immigration route.

Britain and Canada also updated their travel advice for Hong Kong, saying there was an increased risk of detention.

British Foreign Secretary Dominic Raab described Wednesday’s protests as heartbreaking and reprimanded HSBC and other banks for supporting the new law, saying the rights of Hong Kong should not be sacrificed for bankers’ bonuses.

-AAP

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