Fourteen democracy activists in Hong Kong were convicted on Thursday on national security charges, adding to the ranks of dozens of others — once the vanguard of the city’s opposition — who may now become a generation of political prisoners.

The authorities had accused 47 pro-democracy figures, including Benny Tai, a former law professor, and Joshua Wong, a protest leader and founder of a student group, of conspiracy to commit subversion. Thirty-one of them had earlier pleaded guilty. On Thursday, judges picked by Hong Kong’s Beijing-backed leader convicted 14 of the remaining activists and acquitted two others. The charge carries a maximum penalty of life imprisonment.

The convictions show how the authorities have used the sweeping powers of a national security law imposed by Beijing to quash dissent across broad swathes of society. Most of the defendants had already spent at least the last three years in detention before the 118-day trial ended.

Some of those accused are former lawmakers who joined politics after Hong Kong was returned to Chinese rule by the British in 1997. Others are activists and legislators who have advocated self-determination for Hong Kong with more confrontational tactics. Several, like Mr. Wong, who rose to fame as a teenage activist, were among the students leading large street occupations in 2014 for the right to vote.

Their offense: holding a primary election to improve their chances in citywide polls.

“The message from the authorities is clear: Any opposition activism, even the moderate kind, will no longer be tolerated,” said Ho-fung Hung, an expert on Hong Kong politics at Johns Hopkins University.

The pro-democracy activists have said they were merely defending the rights of Hong Kong residents in the face of Beijing’s tightening control over the city. Public alarm over shrinking freedoms in Hong Kong had set off enormous, at times violent, protests in 2019 and early 2020, mounting the greatest challenge to Chinese authority since 1989.

In response, China imposed a national security law on Hong Kong in 2020, handing the authorities a powerful tool to round up critics like the 47 people on trial, including Mr. Tai, the law professor who had been a leading strategist for the pro-democracy camp, and Claudia Mo, a former lawmaker and veteran campaigner.

The authorities charged them with “conspiracy to commit subversion” over their efforts in 2020 to organize or take part in an unofficial primary election ahead of a vote for seats on the Legislative Council.

In the past, pro-democracy activists had held primaries, without issue, to select candidates to run in the election of the city’s leader, Professor Hung said.

“The fact that they were arrested and convicted and even put behind bars for so long before the verdict manifests a fundamental change in Hong Kong’s political environment: Free election, even the pretension of a free election, is gone,” Professor Hung said.

The case the Hong Kong authorities had made against the activists is complicated, and based largely on a scenario that hasn’t happened. Prosecutors said the unofficial primary election was problematic because the pro-democracy bloc was using it to win a majority in the legislature, which would then let them attempt to subvert the government. They accuse the activists of plotting to use such a majority to “indiscriminately” veto the government budget and ultimately force the city’s leader to resign.

The judges ruled that the plan, if carried out, would have led to a “constitutional crisis,” amounting to subversion under the national security law.

The authorities postponed the election, citing the pandemic. By the time the vote was held in late 2021, the activists had been arrested and the electoral rules had been rewritten to effectively disqualify pro-democracy candidates.

The trial of the 47 began in February of last year, after lengthy procedural delays.

Of the defendants, 31 entered guilty pleas, including Mr. Wong, who since 2020 has served prison sentences in other cases related to his activism. Four of them — Au Nok-hin, a former lawmaker; Andrew Chiu and Ben Chung, former district officials; and Mike Lam, a grocery chain owner with political ambitions — testified for the prosecution in exchange for a reduced sentence.

The 14 defendants who were convicted on Thursday included Leung Kwok-hung, a veteran activist known as “Long Hair” who pushed for welfare policies for the old and the poor; Lam Cheuk-ting, an anti-corruption investigator turned legislator; and Gwyneth Ho, a former journalist. The two defendants who were acquitted were Lawrence Lau, a lawyer, and Lee Yue-shun, a social worker. (Prosecutors said they intended to appeal the acquittals.)

After the verdict was handed down, family members of some of the defendants embraced each other and cried outside the courtroom. Johnson Yeung, a rights activist who attended the hearing, said that he was pained by the verdict, even though it was one that he and many others had expected.

“I thought about the face of each of the defendants and the friends remaining in jail. That made me sad. That made me want to hug them,” he said.

Since the activists were arrested en masse, the city has all but eliminated opposition voices in its political institutions. Only approved “patriots” were allowed to run for election to the city’s legislature in 2021. In March, Hong Kong passed its own national security law with extraordinary speed, at the behest of Beijing.

On Tuesday, the city used the new homegrown security law for the first time, detaining six people, including the activist Chow Hang Tung, for allegedly publishing “seditious materials” online. (Ms. Chow is already in prison over separate charges.)

Observers say that the political cases are testing the city’s much-vaunted judicial independence. A trial against Jimmy Lai, a media tycoon and an outspoken critic of Beijing, is underway. Weeks ago, a court granted a government request to ban a popular protest song, raising concerns about speech.

The drawn-out legal process in the trial of the 47 democrats and lengthy detention have come at a heavy personal cost for the defendants. One former legislator, Wu Chi-wai, lost both parents while behind bars. Many of the defendants are parents of young children.

“Almost all of them are seeing their own lives being put on hold — these are some of the best and brightest of Hong Kong, all of whom have seen their careers cut short as they endure month after month behind bars,” said Thomas Kellogg, the executive director of the Georgetown Center for Asian Law. “A truly sad story.”

Some activists have sought to highlight what they see as the resilience of the defendants over the past years. Owen Chow, a 27-year-old activist who was among those convicted, had studied Buddhist scriptures and the writings of Thich Nhat Hanh, a Vietnamese monk and peace activist, while in jail, according to his friends, who posted updates about him on social media on his behalf. And Mr. Lee, the social worker who was acquitted, wrote on Facebook that he had earned a law degree while on bail.

“This shows that whether you are in prison or outside prison, no matter what situation you are in, you can find something meaningful to do, even when there are a lot of restrictions,” said Mr. Yeung, the activist who attended the hearing.

During sentencing, which will take place at a later date, the defendants are expected to be sorted into tiers, legal scholars have said. Those considered “principal offenders” could be sentenced to between 10 years and life imprisonment. “Active participants,” between three and 10 years in prison. Others who are found guilty could be imprisoned or subject to unspecified “restrictions” for up to three years.