HONG KONG (Reuters) – Hong Kong police said on Wednesday they arrested a man holding a flag advocating for independence, the first arrest under a sweeping national security law Beijing introduced overnight.

Pictures posted by police on Twitter showed the flag lying on the sidewalk in front of a man wearing a black T-shirt with “Free Hong Kong” written on it.

Beijing on Tuesday unveiled the details of the much-anticipated law after weeks of uncertainty, pushing China’s freest city and one of the world’s most glittering financial hubs onto 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 to make arrests, while shops and one metro station closed.

“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.

Crowds spilling out into the streets chanted “resist till the end” and “Hong Kong independence”.

Police fired water cannon to chase them away and later said they had made 30 arrests for illegal assembly, obstruction, possession of weapons and violating the new law.

Earlier, police cited the law for the first time 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.

The law will punish crimes of secession, subversion, terrorism and collusion with foreign forces with up to life in prison and officially set up mainland security agencies in Hong Kong for the first time, with powers beyond city laws.

China’s parliament adopted it in response to months of pro-democracy protests last year triggered by fears that Beijing was stifling the city’s freedoms, 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 will crush the freedoms that are seen as key to Hong Kong’s success as a financial centre.