Outside pressures also played a role in the business leaders’ speaking out. Local tycoons were spooked by rising threats by lawmakers in the United States to reconsider an American policy granting special trade status to Hong Kong, raising the prospect that the city could become another front in President Trump’s trade war with China.

Hong Kong, a former British colony, owes its rise to its position as the world’s bridge to China. It operates under its own laws, enjoying independent courts and wide freedom of expression. That has made it an ideal base for local billionaires and global companies to run their mainland businesses.

Those business types have long enjoyed tremendous power. They control roughly one-third of the seats of the Legislative Council, the city’s lawmaking body. A few tycoons own the property companies, utilities, supermarkets and other businesses central to everyday life.

Their support, or lack of it, has helped decide the fate of other political movements. A pro-business party effectively blocked a 2003 law that would have extended China’s national security apparatus into the city, resulting in the eventual ouster of Hong Kong’s top leader. By contrast, businesses harshly criticized the 2014 occupation of parts of the city by protesters opposing the mainland’s increasing sway in Hong Kong. The protest ultimately faltered.

The extradition bill drew immediate worry from the business community when it was submitted in February. The bill would make it possible for Hong Kong to extradite suspects to the mainland. In closed-door meetings, business leaders argued that it could make them vulnerable to charges of corruption in China, where bribery has long been seen as a cost of doing business but has become the target of a nationwide crackdown in the past few years.

“When we started to open up factories in China, the overall rule of law was not so mature,” said Felix Chung , a Legislative Council member and Liberal Party leader who represents the textiles and garment sector.

“A lot of things had to be done by special ways, through corruption, bribery or whatever,” said Mr. Chung, who opened his factory on the mainland in 1993.