The answer, the letter says, lies in ensuring that Hong Kong’s elections are open, without Beijing’s wielding power to engineer the outcome by excluding candidates or skewing the voting procedures. China’s national legislature has said that the election of the Hong Kong chief executive in 2017 “may be implemented by the method of universal suffrage,” but critics say the Hong Kong government’s proposals to deliver that broad promise could be fatally compromised by limits demanded by Beijing. Hong Kong’s chief executive is currently chosen by an elite committee of about 1,200 people, many of them seen as beholden to the Chinese government.

Mr. Chin and his supporters, who call themselves a financial arm of the “Occupy Central” movement, embody a trend that could prove increasingly troublesome for Beijing’s efforts to manage Hong Kong, which maintains its own administration and laws. Anxiety over mainland influence is reaching up the social ladder to include members of the professional establishment who outwardly appear to have much to gain from avoiding confrontation with the Chinese government and its supporters. Critics of Occupy Central warn that the movement could hurt economic confidence, but its backers say Hong Kong’s long-term economic health requires political action to press for change.

“Most people think people who work in banking or finance only focus on work, or people say we are the beneficiary of this system,” said Lai Chong Au, a marketing manager at an investment firm, who has joined the group started by Mr. Chin.

“But that’s not true,” she said. “In the long run, if you want to maintain an international banking and financial center in Hong Kong, you need to have a good system, a good framework, in order to protect it.”

Michael E. DeGolyer, a political scientist at Hong Kong Baptist University, said a forthcoming study of public opinion showed levels of public discontent with the Hong Kong government similar to those seen before mass protests in 2003, when the government tried to introduce Beijing-backed antisubversion laws.