Hong Kong’s highest court started its first trial over former media mogul Jimmy Lai’s role in pro-democracy protests in 2019 after a spate of departures of foreign judges renewed scrutiny on the city’s judiciary.

The Court of Final Appeal on Monday began hearing the appeal from Lai and six former lawmakers who seek to overturn their conviction for joining an unlawful protest in August 2019. A lower court earlier cleared the seven pro-democracy activists of a charge of organizing that rally but upheld their guilty verdict for participating in it.

The hearing comes days after a prominent U.K. judge, Jonathan Sumption, resigned from the top court in one of three exits announced this month and warned of a "grave danger" to the rule of law in the finance hub because of Beijing’s tightening grip.