New Delhi, Jan. 22 -- A trial against two pro-democracy activists behind a group that for decades organized a vigil that commemorated people killed in Beijing's Tiananmen Square crackdown in 1989 opened Thursday, in another landmark case brought under a China-imposed national security law that has practically crushed protests in the semiautonomous Chinese city.

Critics say their case shows that Beijing's promise to keep the city's Western-style civil liberties intact for 50 years when the former British colony returned to Chinese rule in 1997 has weakened over time. But the city's government said its law enforcement actions were evidence-based and strictly in accordance with the law.

Chow Hang-tung and Lee Cheuk-yan, former leaders of t...