MOSCOW—Russian police this week raided the offices of a prominent environmental group that had protested government plans to reopen a controversial paper mill on the shores of Siberia's Lake Baikal.

Marina Rikhvanova, head of Baikal Ecological Wave, said local police in the Siberian city of Irkutsk had seized the group's computers in a raid, saying they sought suspected pirated software. Ms. Rikhvanova denied the allegations and said she suspected the authorities were trying to thwart her group's efforts to organize a demonstration...