For most of my life I have advocated change in the world’s most closed, repressive nation: North Korea. As a South Korean, my priority has been peace and freedom on the peninsula I call home. But I cannot ignore the erosion of freedom in what was once one of Asia’s freest cities: Hong Kong.

There used to be parallels between South Korea and Hong Kong. Both are economic “tigers,” with basic freedoms and an openness to the world, living in the shadow of authoritarian regimes to the north.

But now there is a divergence, as Hong Kong is being subsumed into Xi Jinping’s China in the midst of its worst crackdown on human rights since the Tiananmen Square massacre 30 years ago.

Last week Hong Kong authorities handed prison sentences to nine leaders of the pro-democracy Umbrella Movement. This is merely the latest alarming example of suppression in the city, in which civil rights are steadily declining.

To be sure, Hong Kong cannot be compared to totalitarian North Korea. Though Hong Kongers are denied universal suffrage, there is still some freedom of expression and a generally fair legal system. Since Britain handed over control to China in 1997, Hong Kong has functioned under the principle of “one country, two systems” with a high degree of autonomy. It was for years the only place in China where people could demonstrate, speak out in the press, or criticize the government without fear of arrest. All that is now in jeopardy.

Over the past five years Hong Kong has faced a series of threats to basic freedoms, from the abduction of booksellers to the disqualification of pro-democracy legislators and candidates. We have seen the expulsion of journalist Victor Mallet, British human-rights activist Benedict Rogers and Japanese politician Kenichiro Wada. Peaceful pro-democracy protesters have increasingly been jailed, including the youngest elected legislator, Nathan Law, and student leader Joshua Wong.

Most alarming is the way the law is being turned against dissidents. A new law will make it a crime to insult China’s national anthem, even though Hong Kong’s Chief Executive Carrie Lam admits that it’s “very difficult” to define the offense. A national-security law, with an “antisubversion” clause, will make civil society and democratic opposition extremely hazardous.

Most dangerous is the proposed change to extradition laws that would allow Hong Kong to send suspected criminals to mainland China. Activists cite the 2015 abduction of five booksellers as an example of the type of government kidnapping that could be legalized under such an extradition law. If it passes, Hong Kong residents could be transferred to the mainland and subjected to a judicial system that has no independence and functions at the whim of the Communist Party. Domestic and foreign businesspeople also have reason to be concerned. Yet despite warnings from Hong Kong’s Law Society, the American Chamber of Commerce, and the U.S. and other governments, Ms. Lam insists it must be implemented.

A recent report by Hong Kong Watch rightly argued that “what is in need of protection in Hong Kong today is not the national anthem, but rather such basic rights as freedom of opinion, expression, protest and association.” Last weekend thousands of Hong Kongers protested the new proposals. Chris Patten, Hong Kong’s last colonial governor in the 1990s, observed that “societies which believe in the rule of law do not reach agreements like this with those who do not.” He describes the proposed new extradition arrangement as “an assault on Hong Kong’s values, stability and security,” adding that the law would make it more difficult for the rest of the world to believe that “Beijing can be trusted to keep its word.” He’s right.

Hong Kong has long been a center for human-rights groups and media in the region, as well as a window into China. It is also one of the world’s most important financial centers, a dynamic hub of entrepreneurship and free trade. Businesses and political advocacy groups may flee a city where their rights are no longer guaranteed.

The crackdown in Hong Kong is a cause for international concern. The U.K. still carries particular responsibilities under the Sino-British Joint Declaration, and the world rightly looks to Britain to lead, but like-minded countries must also defend liberty in Hong Kong. As a citizen of a relatively young East Asian democracy—with the other half of our peninsula living under a barbaric totalitarian regime—I believe it is time to defend Asia’s world city, and make sure it continues to be worthy of that name.

Mr. Lee is South Korea’s former ambassador for human rights, a professor of international relations at Yonsei University in Seoul, and an international patron of Hong Kong Watch. The piece was originally published in The Wall Street Journal on 2 May 2019.