Hong Kong police have made their first arrests under a new "anti-protest" law imposed by Beijing, as crowds marked 23 years since the end of British rule.
Police say two people were held for breaching the new law, including a man holding a pro-independence flag. Dozens of others were arrested during a rally.
The national security law targets secession, subversion and terrorism with punishments up to life in prison.
Activists say it erodes freedoms but China has dismissed the criticism.
Hong Kong's sovereignty was handed back to China by Britain in 1997 and certain rights were supposed to be guaranteed for at least 50 years under the "one country, two systems" agreement.
On Wednesday, crowds gathered for the annual pro-democracy rally to mark the anniversary, defying a ban by authorities who cited restrictions on gatherings of more than 50 people because of Covid-19.
Police used water cannon and pepper spray on demonstrators and said at least 70 people had been arrested, two of them under the new security law.
It said one of them included a man who was holding a "Hong Kong Independence" flag, though some Twitter users said the picture appeared to show a small "no to" written in front of the slogan, Reuters reported. The man has not been identified, and it was not clear whether he would be prosecuted.
The legislation has been widely condemned by countries including the US and UK as well as human rights activists. US Secretary of State Mike Pompeo said: "[China] promised 50 years of freedom to the Hong Kong people, and gave them only 23."
Speaking in Beijing, Chinese Foreign Ministry spokesman Zhao Lijian urged countries to look at the situation in Hong Kong objectively and said China would not allow foreign interference in its domestic affairs.
Earlier, Zhang Xiaming of the Hong Kong and Macau Affairs office of the State Council, bristled at foreign critics, asking them: "What's this got to do with you?"
Chief Executive Carrie Lam, Hong Kong's pro-Beijing leader, said the law would "restore stability" after widespread protests in 2019, saying: "The [new law] is considered the most important development in relations between the central government and Hong Kong since the handover."
Meanwhile, UK Foreign Secretary Dominic Raab will make a statement later with the country's response, after accusing China of ignoring international obligations by taking new powers that could curb civil liberties.
Hong Kong's new security law
What is happening on the anniversary?
Demonstrators in the Causeway Bay district chanted "resist till the end" and "Hong Kong independence" amid warnings that certain slogans and banners might constitute serious crimes under the controversial law.
"I'm scared of going to jail but for justice I have to come out today, I have to stand up," a 35-year-old man who gave his name as Seth told Reuters news agency.
Ahead of the protest, pro-democracy activist Tsang Kin-shing, of the League of Social Democrats, warned there was a "large chance of our being arrested", saying: "The charges will not be light, please judge for yourself."
Photos on social media - confirmed by police as genuine - showed a flag being used by officers to warn protesters about the new law. Some 4,000 officers were on standby to handle unrest, the South China Morning Post reported.
Image copyright Twitter Image caption Police are carrying this banner warning protesters about the new lawWhat does the new law say?
Crimes of secession, subversion, terrorism and collusion with foreign forces are punishable by a minimum sentence of three years, with the maximum being life. It also says:
Damaging public transport facilities - which often happened during the 2019 protests - can be considered terrorism Beijing will establish a new security office in Hong Kong, with its own law enforcement personnel - neither of which would come under the local authority's jurisdiction Inciting hatred of China's central government and Hong Kong's regional government are now offences under Article 29 The law can also be broken from abroad by non-residents under Article 38, and this could mean that foreigners could be arrested on arrival in Hong Kong if they are suspected of breaking the new law Some trials will be heard behind closed doorsBeijing will also have power over how the law should be interpreted, and not any Hong Kong judicial or policy body. If the law conflicts with any Hong Kong law, the Beijing law takes priority.
Mr Zhang said the law would not be applied retroactively - that is, to offences committed before it was passed - and that suspects arrested in Hong Kong on charges of violating the law may be tried on the mainland.
A turning point for Hong Kong
By Michael Bristow, BBC World Service Asia-Pacific editor
The law gives Beijing extensive powers to shape life in the territory that it has never had before. It not only introduces a series of tough punishments for a long list of crimes, it changes the way justice is administered.
Trials can be held in secret - and without a jury. Judges can be handpicked. The law reverses a presumption that suspects will be granted bail. There appears to be no time limit on how long people can be held.
Crimes are described in vague terms, leading to the possibility of broad interpretation, and the right to interpret lies only in Beijing. Foreign nationals outside of Hong Kong face prosecution.
Most cases will be handled in Hong Kong, but the mainland can take over "complex", "serious" or "difficult" cases. Whether or not you think the legislation was necessary, it is impossible to deny its significance. As Hong Kong's leader Carrie Lam put it: this is a turning point.
What reaction has the new law drawn?
Minutes after the law was passed on Tuesday, pro-democracy activists began to quit, fearful of the punishment the new law allows.
"With sweeping powers and ill-defined law, the city will turn into a secret police state," said Joshua Wong, a key pro-democracy leader. The political party he co-founded - Demosisto - was disbanded.
Ted Hui, an opposition legislator, told the BBC the move had taken away the city's rights, saying: "Our freedom is gone, our rule of law, our judicial independence is gone".
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Mr Pompeo said the "draconian" law "destroyed Hong Kong's autonomy", adding: "Beijing's paranoia and fear of its own people's aspirations have led it to eviscerate the very foundation of the territory's success."
Canada has updated its Hong Kong travel advice, saying: "You may be at increased risk of arbitrary detention on national security grounds and possible extradition to mainland China."
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In the US, lawmakers from both parties have launched a bill to give refugee status to Hong Kong residents at risk of persecution, reported local media outlets.
Taiwan's government has said it will set up a special office to help those in Hong Kong facing immediate political risks.