Hong Kong police have made their first arrest under a new national security law, arresting a protester for carrying a flag calling for Hong Kong independence at a protest Wednesday in the city’s Causeway Bay shopping district.

The man was intercepted by police and arrested after police had issued multiple warnings to the crowd that they might be in violation of the national security law, which took effect Tuesday at 11 p.m. (1500 GMT).

The law makes secessionist, subversive or terrorist activities illegal, as well as foreign intervention in the city's internal affairs. It was imposed by China after months of anti-government protests in the semi-autonomous territory last year.

Hong Kong’s leader strongly endorsed the new security law China's central government is imposing on the semi-autonomous territory in her speech marking Wednesday's anniversary of its handover from colonial Britain.

"This decision was necessary and timely to maintain Hong Kong’s stability," Carrie Lam said following a flag-raising ceremony and the playing of China's national anthem.

A pro-democracy political party, The League of Social Democrats, organized a protest march during the flag-raising ceremony preceding Lam’s speech. 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.

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, while anyone taking part in activities deemed as secessionist would also be in violation of the new law.

The law takes immediate effect and its consequences could come swiftly. Hong Kong's police force issued a statement saying they would consider as illegal any flag or banner raised by protesters deemed to be promoting Hong Kong's separation from China or expressing support for independence for Tibet, Xinjiang and the self-governing island democracy of Taiwan that China claims as its own.

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 the Legislative Council elections scheduled for September.

Under the law, those found guilty of inciting secessionist, subversive, terrorist activities and colluding with foreign forces could face life imprisonment if they are deemed masterminds of such activities.

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

It 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, according to the text.

The 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 in the face of massive public opposition. Having lost patience, Beijing finally decided to circumvent the Hong Kong legislature and have it 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 U.S. has already begun moves 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, citing the possibility of them falling into the hands of the People’s Liberation Army, which owes its loyalty to China's ruling Communist Party.

Congress has also moved to impose sanctions on people deemed connected to political repression in Hong Kong, including police officials, while Britain has said it could offer residency and possible citizenship to about 3 million of Hong Kong’s 7.5 million people.

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.

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. Pro-China figures have also been pushing for more "patriotic" education to be introduced into the curriculum in hopes that will boost their identification with Beijing.