HONG KONG — Tens of thousands of people marched under a blistering sun in Hong Kong on Sunday to express their opposition to a pro-democracy movement that has threatened to bring Asia’s biggest financial center to a standstill if the government does not open up the nomination process for electing the city’s top leader.

Protesters, many waving Chinese flags, streamed into Victoria Park in the midafternoon before the march, and the contrast with a rally held July 1 by pro-democracy organizers was stark. Most of the participants in Sunday’s rally were organized into groups corresponding to Chinese hometowns, schools or, in some cases, employers, easily identifiable with their matching T-shirts and hats. Middle-aged and elderly people dominated Sunday’s march, while young people dominated last month’s march.

In speech, they often employed the political lexicon of China’s ruling Communist Party. Typical was Kitty Lai, an investment adviser wearing an orange T-shirt and a baseball cap emblazoned with the logo of the Hong Kong Federation of Fujian Associations, a group that represents people from the coastal province across from Taiwan. She said shutting down the Central business district would cause chaos.

“We want everything to be stable,” Ms. Lai, 50, said in Mandarin Chinese. “We want everybody to live harmoniously.”