To a Hong Kong national security court, Jimmy Lai’s only intent was to “seek the downfall” of China’s ruling Communist Party. To Hong Kong’s leader, Mr. Lai’s actions were “malicious.” And to the police, his guilty verdicts on Monday amounted to “justice served.”

But to the small group of supporters who gathered quietly outside the courthouse on Monday before the verdict, the media tycoon and outspoken critic of Beijing was someone who stood up for them. He was also someone who joined pro-democracy protesters in the streets of Hong Kong until 2020 when China imposed an expansive national security law.

In a sign of how much things have changed in this city since, Mr. Lai’s supporters were reluctant to speak to reporters. Many wore masks to cover their face. One supporter ushered a New York Times reporter to a corner away from police, who stood nearby looking on, before saying that the landmark trial marked the end of a chapter for the once-freewheeling media outlets in the city. Another supporter said she had attended every single hearing, of which there have been more than 150, over the past two years.

The verdicts — which she correctly assumed would be guilty — were a watershed moment, she said. Who would stand up for the beliefs of people like her now, she wondered.

Presided over by a panel of three judges handpicked by Hong Kong’s leader, the trial did not have a jury, in a break from the city’s traditional common-law system that applies to national security cases.

Esther Toh, one of the three judges, read from the concluding remarks of an 855-page ruling for nearly 40 minutes on Monday.

Dressed in a light-colored blazer and wearing black-rimmed glasses, Mr. Lai, who turned 78 last week, looked ahead as she read the decision that he had been found guilty on two counts of “conspiracy to collude with foreign forces,” in part over meetings he had held with politicians in the United States, and one count of conspiracy to publish seditious material in Apple Daily, the now-shuttered Chinese-language newspaper he founded in 1995.

There was “no doubt” that Mr. Lai’s actions were motivated by “resentment and hatred” for China, Judge Toh said. She described Mr. Lai as “gleeful” about the prospect of the U.S. government imposing sanctions on the city after the authorities had suppressed monthslong mass antigovernment protests that erupted in 2019.

The judges painted Mr. Lai as a wealthy man who used his money and influence to lobby foreign countries, and, in particular, the United States. These actions, they said, were akin to an American national working with Russia to bring down the U.S. government under the banner of helping the state of California.

From inside a glass box in the courtroom, Mr. Lai sat mostly still as he listened.

The self-proclaimed “troublemaker” — whose Apple Daily once gave blanket coverage to the pro-democracy demonstrations — now faces up to life in prison. The court has set four days aside for a pre-sentencing hearing starting on Jan. 12 and the punishment would be announced “as soon as possible,” Judge Toh said.

After hearing the verdict, Mr. Lai stood up and waved to his wife, Teresa, who was sitting with one of his sons, Lai Shun Yan, and Cardinal Joseph Zen, the former Roman Catholic bishop of Hong Kong and an outspoken supporter of democracy.

Mr. Lai was already behind bars for five years, having previously been convicted of fraud for violating the terms of a lease agreement. Long known for his broad features, he now appears much slimmer. His family says his health is deteriorating from being held in solitary confinement, a point that police disputed in a news conference following the hearing on Monday.

Hong Kong officials say Mr. Lai is receiving “adequate and comprehensive” medical care and that he had requested to be held in solitary confinement.

International human rights groups condemned the guilty verdict as further evidence of the wider crackdown on free speech in Hong Kong. Sarah Brooks, Amnesty International’s China director, said it showed how the city’s national security laws were designed to “silence” its people.

Mr. Lai’s verdict comes as Hong Kong is still dealing with the aftermath of its most deadly fire in decades, an apartment complex blaze that killed at least 160 people last month. The national security police have made more than a dozen arrests since the fire and have been on heightened alert for what they say are “anti-China forces” looking to exploit the disaster to undermine social stability. Critics have said the authorities are targeting people calling for greater government accountability.

Mr. Lai’s verdicts will bring attention to what analysts say is Hong Kong’s shrinking tolerance for dissent against the government and free speech, as well as the continued erosion of the “one country, two systems” framework that was supposed to guarantee the city some autonomy from Beijing until 2047.

The political changes have been largely driven by two national security laws, one imposed by Beijing in 2020 and another law introduced by the Hong Kong government in 2024 that expanded its scope and specifically targeted treason, secession, subversion and sedition. Both laws were largely a response to the widespread and sometimes violent demonstrations that engulfed the Asian financial center in 2019.

They have served to muzzle much of the once-open forum for debate and discussion in Hong Kong, according to experts.

Before the hearing on Monday morning, the police were on high alert. As the sun began to rise, dozens of officers jumped out of police vans, spilling into the streets and onto the sidewalks. Officials herded reporters and cameramen into two pens several hours before the trial began. Two officers walked around with bomb-sniffing dogs, and an anti-riot armored vehicle from mainland China, known as the “saber-toothed tiger,” rolled by.

Emily Lau, a former chair of the Democratic Party, who attended the hearing on Monday, said she could not comment on the verdict. But she had a message for Hong Kong residents who felt impacted by the political changes that have swept through the city in recent years, including the disbanding of the Democratic Party, Hong Kong’s largest opposition party, on Sunday.

“We have to be tough to survive in such an environment,” said Ms. Lau. “We can see the authorities keep giving people examples of just how tough things are, but life has to go on.”