HONG KONG — In the latest sign of Communist China’s crackdown on civil liberties in Hong Kong, 14 pro-democracy activists were convicted in the former British colony’s biggest national security case on Thursday by a court that said their plan to hold an unofficial primary election would have undermined the government’s authority and created a constitutional crisis.

After a 2019 protest movement that filled the city’s streets with demonstrators, authorities have all but silenced dissent in Hong Kong through reduced public choice in elections, crackdowns on press, and the Beijing-imposed security law under which the activists were convicted.

Those found guilty of conspiracy to commit subversion included former lawmakers Leung Kwok-hung, Lam Cheuk-ting, Helena Wong, and Raymond Chan, and they could face up to life in prison when sentenced later. The two defendants acquitted were former district councilors Lee Yue-shun and Lawrence Lau. But the prosecution said it intends to appeal against the acquittals.

The activists were among 47 democracy advocates who were prosecuted in 2021 for their involvement in the primary. Prosecutors had accused them of attempting to paralyze Hong Kong’s government and topple the city’s leader by securing the legislative majority necessary to indiscriminately veto budgets.

Observers said the subversion case illustrated how the security law is being used to crush the political opposition following huge anti-government protests in 2019. It also showed that Beijing’s promise to retain the former British colony’s Western-style civil liberties for 50 years when it returned to China in 1997 was becoming increasingly threadbare, they said.

The Beijing and Hong Kong governments, however, say the law has helped bring back stability to the city and that judicial independence was being protected. After the verdicts, Beijing voiced its support for the work of the city’s judicial and law enforcement officials, despite concerns from the West.

In a summary of the verdict distributed to press, the court said the election participants had declared they would use their legislative power to veto the budgets.

Under the city’s mini-constitution, the chief executive can dissolve the legislature if a budget cannot be passed but the leader would have to step down if the budget is again vetoed in the next legislature.

In the full, 319-page verdict, the judges approved by the government to oversee the case also said if the plan to veto bills would lead to the dissolution of the legislature, it meant “the implementation of any new government policies would be seriously hampered and essentially put to a halt.”

“The power and authority of both the Government and the Chief Executive would be greatly undermined,” the court said in the verdict. “In our view … that would create a constitutional crisis for Hong Kong.”

As the hearing concluded, some of the convicted defendants waved at their families as they left the courtroom.

The court acquitted Mr. Lau after it found he had not mentioned vetoing the budget in his election campaign and the court was unable to conclude he had intended to subvert state power.

Mr. Lee, the other defendant found not guilty, thanked the public for caring about the case over the past few years. “I feel calm, as I have always been,” he said.

Mr. Lee, like Mr. Lau, was acquitted after the court found no evidence he mentioned vetoing in an election forum, nor had he personally expressed his stance on using veto power to force the government to accede to the 2019 protest demands.

While Mr. Lee had adopted a similar political platform as other party members in the now-defunct Civic Party, the court took into account that he was a latecomer to the party’s campaign for the primary and that he would have had little choice but to adopt the platform used by others. Thus, the court said it could not be sure he had intended to subvert state power.

The two will be kept on bail pending appeal, the court said. A mitigation hearing has been tentatively scheduled for June 25.

The 47 activists charged included legal scholar Benny Tai, former student leader Joshua Wong, and a dozen former lawmakers including Leung and Claudia Mo.

Thirty-one of them, including Mr. Tai, Mr. Wong and Ms. Mo, pleaded guilty. They have a better chance at shorter jail terms and will be sentenced at a later date.

Before the court hearing began on Thursday, four members of pro-democracy party League of Social Democrats, including Mr. Leung’s wife Chan Po-ying, were arrested outside the court building, according to a Facebook post by party member Figo Chan. They were initially planning to stage a tiny protest to voice support for the activists.

Diplomats from the United States, Australia and Britain, along with dozens of residents had waited outside the police-guarded court building to secure seats to hear the verdicts. Rights groups and several foreign governments later criticized the court’s decision.

The unofficial primary in June 2020 was meant to shortlist pro-democracy candidates who would then run in the official election. It drew an unexpectedly high turnout of 610,000 voters, more than 13 percent of the city’s registered electorate.

The pro-democracy camp at that time hoped they could secure a legislative majority, which would allow them to press for the 2019 protest demands, including greater police accountability and democratic elections for the city leader.

The government postponed the legislative election that would have followed the primary, though, citing public health risks during the coronavirus pandemic. The electoral laws were later overhauled, effectively increasing the number of pro-Beijing lawmakers in the legislature.