A Hong Kong pro-democracy activist, who was detained in mainland China after trying to flee the city by boat, was found guilty on Thursday of conspiracy to commit collusion with a foreign country in a national security case.

Andy Li was among a group of 12 people intercepted by mainland authorities in August 2020 on a boat believed to be en route to Taiwan, in a case that drew international attention and concern over their treatment in China.

The court heard Li, 31, was...