In the first trial under the tough national security law imposed by China, a Hong Kong court convicted a protester of terrorism and inciting secession on Tuesday, a ruling that sets new limits on dissent and raises the risks of challenging Beijing.

       The protester, Tong Ying-kit, was arrested last year on July 1, after he drove his motorcycle around a Hong Kong neighborhood with a protest banner, then collided with police officers who tried to stop him, injuring three.

       Under the security law, which went into effect hours before his arrest, Mr. Tong could be sentenced to life in prison. The court will hand down his sentence at a later date.

       The trial was regarded as a test of how the city’s traditionally independent courts would enforce the security law while upholding the city’s much cherished civil liberties. In convicting Mr. Tong, the court showed the extent to which the new law would criminalize political speech.

       “I think we would be kidding ourselves if we think the national security law would not be enforced in the way the Chinese government conceived it,” said Steve Tsang, director of the SOAS China Institute in London. “The idea that the Hong Kong judiciary would be able to moderate it would be a mistake.”

       Under the provisions of the law, Mr. Tong was denied bail and a jury trial. Instead, he was tried by a panel of three judges, assigned from a group selected by Hong Kong’s chief executive, Carrie Lam.

       The authorities have already used the law, which vaguely defines political crimes such as subversion and terrorism, to curb protests and arrest leaders of the pro-democracy movement.

       More than 60 other people are awaiting trial on charges brought under the law. They include dozens of pro-democracy politicians who have been accused of subversion for their calls to block the government’s agenda in the legislature. As their cases go to trial, the courts will be closely watched for signs of how the law may erode the city’s much vaunted British common law traditions of fairness and independence.

       The Hong Kong authorities have used the law as a cudgel against those participating in protests, political campaigns and expressions of opinion, a dramatic change for a city that has long had some of the strongest protections for speech in Asia.

       In Mr. Tong’s case, the court agreed with the authorities that a popular protest slogan on his banner, “Liberate Hong Kong, Revolution of Our Times,” amounted to a call for independence from China, which is banned under the law.

       “The defendant himself understood the slogan to carry a secessionist meaning,” the court said in its ruling.

       Rights activists said the verdict would significantly limit free speech in the city. The slogan on his banner was widely used during the protests that convulsed the city in 2019, spray-painted on walls, emblazoned on flags and chanted by students and demonstrators on the streets, in malls and outside government buildings.

       “To convict Tong Ying-kit of ‘secession’ for displaying a flag bearing a widely used political slogan is a violation of international law, under which expression must not be criminalized unless it poses a concrete threat,” said Yamini Mishra, Amnesty International’s Asia Pacific director. “This feels like the beginning of the end for freedom of expression in Hong Kong.”

       The prosecution had spent much of the 15-day trial analyzing the meaning of the slogan, relying on the testimony of Lau Chi-pang, a history professor at Lingnan University in Hong Kong, that the meaning of the Chinese words for “liberate” and “revolution” had remained constant for more than 1,000 years.

       The discussion covered a range of history, from the ancient Shang and Zhou dynasties to the Cultural Revolution and Malcolm X. Professor Lau testified that the slogan, which was coined in 2016 by Edward Leung, a now-imprisoned pro-independence activist, called for acts of violence to end China’s control of Hong Kong.

       Mr. Tong’s attorneys had argued that the slogan was open to interpretation.

       “Just as if somebody says let’s go fight for our rights, that doesn’t necessarily mean get out a gun and start shooting people,” said Clive Grossman, Mr. Tong’s lead defense attorney.

       Two defense witnesses, Eliza Lee, a professor of politics at the University of Hong Kong, and Frances Lee, a professor in the journalism school at the Chinese University of Hong Kong, testified that the phrase had carried a variety of meanings in recent years, including a general call for fundamental change.

       The authorities had also accused Mr. Tong of committing terrorism because he had crashed into police officers who had tried to stop him. The court ruled that Mr. Tong’s collision into the officers “was a deliberate challenge mounted against the police, a symbol of Hong Kong’s law and order.”

       “The defendant carried out those acts with a view to intimidating the public in order to pursue political agenda,” the judges wrote.

       Mr. Grossman, the defense lawyer, acknowledged that Mr. Tong should have stopped when ordered to do so by officers, but he said that dangerous driving did not amount to terrorism. He argued that Mr. Tong had at first swerved to avoid the officers, and had braked once, but might have been distracted when at least one officer threw a shield.

       “A person who sets out to commit the act of terrorism by driving into people does not put his foot on the brake,” Mr. Grossman said.