Thousands of protesters take part in a march against an extradition bill in Hong Kong on Sunday. (Jerome Favre/EPA-EFE/Shutterstock)

IN 2003, a proposed anti-subversion law that threatened civil liberties brought hundreds of thousands of protesters to the streets of Hong Kong, which prizes the rule of law and freedom of expression it has retained since returning to Chinese rule in 1997. Authorities backed down. But that was in another era, before the rise to power in Beijing of Xi Jinping, who aspires to construct a 21st-century model of rigid authoritarianism.

On Sunday, Hong Kong saw what appears to have been the biggest demonstration since 2003: Up to 1 million people were reported to have choked the streets to protest a bill that would allow extraditions from Hong Kong to mainland China. The measure could, and probably would, be used to silence critics of Communist Party rule, and it would undermine a legal system whose impartiality has been one of the foundations of the city’s economic success.

Yet even many of those who joined the protest acknowledged they did not expect to succeed in stopping the measure. Hong Kong’s authorities are no longer responsive to public opinion, even when massively expressed. They cater only to Mr. Xi. “We are still doing it,” the territory’s chief executive, Carrie Lam, confirmed on Monday.

Ms. Lam began promoting the extradition measure in March, using as her pretext the case of a Hong Kong man who is accused of murdering a woman in Taiwan, with which Hong Kong does not have an extradition agreement. The law would allow suspects to be transferred on a case-by-case basis to jurisdictions including both Taiwan and the mainland. But Taiwan has already said it will not employ the law, because it treats the island as a part of China.

A much more plausible use would be to transfer Beijing’s opponents, such as the men associated with a Hong Kong-based book publisher who were abducted there and in other countries in 2015 after they marketed gossipy tomes about Chinese leaders. Beijing was heavily criticized for the crude operation and for the subsequent seizure of a businessman who disappeared from a luxury hotel two years ago.

The new law would provide a legal veneer for such kidnappings; though political and business crimes are nominally excluded from the law, communist authorities are adept at concocting criminal charges. One of the men abducted in the book publisher case, Gui Minhai, was first reported to have been charged for a traffic fatality. He continues to be lawlessly imprisoned in China.

The massive crowd in downtown Hong Kong Sunday showed that the city’s residents understand very well what is at stake. Once the law is in effect, politicians, journalists and civil society activists who speak about the city’s steadily eroding autonomy and Mr. Xi’s consolidation of dictatorship will be under perpetual threat. As Jimmy Lai, a local media magnate, put it, “Hong Kong will just become another Chinese city ruled by the Communist Party.”

That, of course, would be a betrayal of China’s commitment to preserve Hong Kong’s separate economic and political systems until 2047. Yet as Mr. Xi has demonstrated again and again, treaties and international law won’t restrain his ambitions. Nor, alas, will the wishes of the Chinese people.