Jimmy Lai, a veteran pro-democracy figure in Hong Kong, pleaded not guilty on Tuesday to national security charges, as prosecutors accused him of colluding with American and other foreign officials to destabilize the city and undermine China’s sovereignty.

Mr. Lai, 76, who founded Apple Daily, a popular antigovernment newspaper, was arrested in 2020, becoming one of the first prominent targets of a national security law imposed by Beijing to crush the opposition. Mr. Lai faces up to life in prison if convicted of national security offenses, and his trial is seen as a test of the independence of the city’s judiciary.

As Mr. Lai, who was wearing a navy blazer and a white shirt, entered the courtroom, he smiled and waved to his wife and two children, who sat in the gallery, before taking his seat in a dock encased by glass.

On Tuesday, the lead prosecutor, Anthony Chau, began making his case by outlining evidence of what he described as Mr. Lai’s collusion with foreign forces, a vaguely defined political crime under the national security law. Mr. Chau’s argument centered on meetings Mr. Lai had with American politicians, messages he had exchanged with officials, interviews he gave to the media, and views that he aired on social media.

He presented a diagram mapping out what he described as Mr. Lai’s foreign political connections, with whom he had met in 2019, including officials in the Trump administration such as the then-vice president, Mike Pence, and secretary of state, Mike Pompeo, as well as other politicians such as Nancy Pelosi, who had been Speaker of the House. Mr. Chau accused Mr. Lai of undermining national security by calling on the American officials to impose sanctions on Hong Kong and Chinese officials responsible for the crackdown on the city’s pro-democracy protesters. Such an act has been deemed a “hostile activity” against the Hong Kong and Chinese governments and is an offense under the national security law.

Mr. Chau described Mr. Lai as a “radical political figure who conspired with others to bring in hatred and stir up opposition.” Mr. Chau said that Mr. Lai’s former employees at Apple Daily and Next Digital, which owns Apple Daily, would testify against him in the trial.

The specific charges against Mr. Lai are “collusion with foreign forces” and a “conspiracy to collude with foreign forces” under the national security law. He also faces a colonial-era sedition charge, relating to publications spanning a two-year period, from April 1, 2019, to June 24, 2021.

Human rights activists as well as the United States and British governments have denounced the charges against Mr. Lai as politically motivated and trumped up to silence a prominent critic of Beijing’s expanding authoritarian grip over the city.

The prosecutor sought to make the case that Mr. Lai had considerable influence in Washington, pointing to messages and WhatsApp groups from Mr. Lai’s phone that purported to show American officials and human rights activist overseas asking for his opinion on action on China.

Mr. Chau also listed as evidence an editorial by Mr. Lai published in The New York Times in 2020, in which he had described the rapid erosion of free speech in the city and listed ways to retaliate against Beijing for its repression. The prosecutor also played video interviews Mr. Lai had with reporters from BBC News, Bloomberg, Fox News and the Financial Times, in which he explained why he was calling on Western governments to impose sanctions.

The prosecution will continue its opening statements on Wednesday. Mr. Lai’s trial started two weeks ago and is expected to run for at least 80 days. He is serving a five-year sentence on a separate conviction of fraud.