Hong Kong police made the first arrests Wednesday under a new national security law imposed by China’s central government, as thousands of people defied tear gas and pepper pellets to protest against the contentious move on the anniversary of the former British colony’s handover to Chinese rule.

Police said 10 people were arrested under the law, including a man with a Hong Kong independence flag and a woman holding a sign displaying the British flag and calling for Hong Kong’s independence — all violations of the law that took effect Tuesday night. Others were detained for possessing items advocating independence.

Hong Kong police said on Facebook that they arrested some 370 people on various charges, including unlawful assembly, possession of weapons and violating the new law, which was imposed in a move seen as Beijing’s boldest step yet to erase the legal firewall between the semi-autonomous territory and the mainland’s authoritarian Communist Party system.

The law, imposed following anti-government protests in Hong Kong last year, makes secessionist, subversive, or terrorist activities illegal, as well as foreign intervention in the city’s internal affairs. Any person taking part in activities such as shouting slogans or holding up banners and flags calling for the city’s independence is violating the law regardless of whether violence is used.

The most serious offenders, such as those deemed to be masterminds behind these activities, could receive a maximum punishment of life imprisonment. Lesser offenders could receive jail terms of up to three years, short-term detention or restriction.

Wednesday’s arrests came as thousands took to the streets on the 23rd anniversary of Britain’s handover of Hong Kong to China in 1997. For the first time, police banned this year’s annual march. Protesters shouted slogans, lambasted police and held up signs condemning the Chinese government and the new security law.

Some protesters set fires in Hong Kong’s trendy shopping district, Causeway Bay, while others pulled bricks from sidewalks and scattered obstacles across roads in an attempt to obstruct traffic. To disperse protesters, police shot pepper spray and pepper balls, as well as deployed water cannons and tear gas throughout the day.

Hong Kong’s leader strongly endorsed the new law in a speech marking the anniversary of the handover of the territory — officially called the Hong Kong Special Administrative Region.

“The enactment of the national law is regarded as the most significant development in the relationship between the central authorities and the HKSAR since Hong Kong’s return to the motherland,” chief executive Carrie Lam said in a speech, following a flag-raising ceremony and the playing of China’s national anthem.

“It is also an essential and timely decision for restoring stability in Hong Kong,” she said.

A pro-democracy political party, The League of Social Democrats, organized a protest march during the flag-raising ceremony. About a dozen participants chanted slogans echoing demands from protesters last year for political reform and an investigation into accusations of police abuse.

The law’s passage Tuesday further blurs the distinction between the legal systems of Hong Kong, which maintained aspects of British law after the 1997 handover, and the mainland’s authoritarian Communist Party system. Critics say the law effectively ends the “one country, two systems” framework under which Hong Kong was promised a high degree of autonomy.

Britain’s foreign secretary, Dominic Raab, told reporters Wednesday the law “is a clear and serious violation” of the Sino-British Joint Declaration, the agreement that paved the way for the former British colony’s handover to Chinese rule.

The law directly targets some of the actions of anti-government protesters last year, which included attacks on government offices and police stations, damage to subway stations and the shutdown of the city’s international airport. Acts of vandalism against government facilities or public transit can be prosecuted as subversion or terrorism.

Pro-democracy lawmaker Claudia Mo said in a news conference that the security legislation does not follow the rule of law and is a dire warning to the free press.

“This would tell you that they want not just to get us, but to intimidate us into inaction, into a catatonic state,” Mo said.

Hong Kong’s police force said they would consider any flag or banner raised by protesters calling for Hong Kong’s separation from China to be illegal as well as an expressions of support for independence for Tibet, Xinjiang or the self-governing island democracy of Taiwan that China claims as its own.

Police will use a new purple flag to warn protesters if they display banners or shout slogans that may constitute a crime under the law.

Concerns have also been raised over the fate of key opposition figures, some of whom have already been charged for taking part in protests, as well as the disqualification of candidates for Legislative Council elections scheduled for September.

In Beijing, the executive deputy director of the Cabinet’s Hong Kong affairs office, Zhang Xiaoming, said Hong Kong people are allowed to criticize the ruling Communist Party but cannot turn those complaints “into actions.”

“What happened recently in Hong Kong has shown a deviation from the right track of the ‘one country, two systems’ (framework),” Zhang told reporters Wednesday. “To some extent, we made this law in order to correct the deviation … to pull it closer to ‘one-country.’”

Schools, social groups, media outlets, websites and others will be monitored and their national security awareness will be raised, according to the law, while the central government will have authority over the activities of foreign non-governmental organizations and media outlets in Hong Kong.

The law says central government bodies in Hong Kong will take over in “complicated cases” and when there is a serious threat to national security. Local authorities are barred from interfering with central government bodies operating in Hong Kong while they are carrying out their duties.

Security legislation was mandated under Hong Kong’s local constitution, but an earlier attempt to pass it in the city’s legislative body in 2003 was shelved because of massive public opposition. Beijing finally decided to circumvent the Hong Kong legislature and have the law passed Tuesday by the Standing Committee of the National People’s Congress, China’s rubber-stamp parliament.

President Xi Jinping signed a presidential order putting the law into effect, and it has been added to the Basic Law, Hong Kong’s constitution.

The law’s passage comes after Hong Kong’s legislature in early June approved a contentious bill making it illegal to insult the Chinese national anthem.

On Wednesday, Raab, the British foreign secretary, announced the UK would extend residency rights for up to 3 million Hong Kong residents eligible for British National Overseas passports to five years from the current six-month limit. After five years, they could apply for settled status and then apply for citizenship 12 months later.

The U.S. is moving to end special trade terms given to the territory. The Trump administration has also said it will bar defense exports to Hong Kong and will soon require licenses for the sale of items that have both civilian and military uses.

The U.S. Congress has also moved to impose sanctions on people deemed connected to political repression in Hong Kong, including police officials.

China has said it will impose visa restrictions on Americans it sees as interfering over Hong Kong.

U.S. Secretary of State Mike Pompeo denounced the threat of a visa ban as a sign of “how Beijing refuses to take responsibility for its own choices” and said the law’s adoption “destroys the territory’s autonomy and one of China’s greatest achievements.”

Beijing’s “paranoia and fear of its own people’s aspirations have led it to eviscerate the very foundation of the territory’s success,” Pompeo said in a statement.

Taiwan on Wednesday opened an office to facilitate migration from Hong Kong.