Suzanne Pepper, a Hong Kong-based scholar of Chinese politics, said the new sentences were “part of a larger pushback by Beijing against Hong Kong’s democracy movement.”

“It’s a two-part strategy aimed at targeting the leaders, making an example of them, showing the cost for all who might want to follow in their footsteps and offering rewards to all who settle down,” she said in an email. “Sort of a combined carrots-and-sticks strategy, plus ‘killing the chicken to frighten the monkeys.’”

Mr. Wong and Mr. Chow were found guilty last year of unlawful assembly, while Mr. Law was found guilty of inciting people to take part in the assembly. The charges stemmed from the storming of a fenced government square in 2014 to protest Beijing’s strict limits on proposed reforms to the way Hong Kong elects its top leader, or chief executive.

The protests and the police response cascaded into weeks of sit-ins, later known as the Umbrella Movement, that paralyzed several major streets across Hong Kong but failed to win the protesters any political concessions.

The former British colony returned to Chinese rule 20 years ago under a “one country, two systems” governing principle that promised a high degree of autonomy. Yet the public’s trust in the political firewall has eroded as the Chinese authorities appear ever more assertive in exercising their will in the city.

Last year, the Chinese government moved to unseat two dissident lawmakers elected in Hong Kong’s only citywide direct elections, ostensibly because they deviated from the official text when taking the oath of office. Last month, four more opposition lawmakers were removed from the city’s 70-member Legislative Council, including Mr. Law, who was elected last year as the city’s youngest-ever legislator.