WARSAW, Poland (AP) — Poland’s ruling party is suing a law professor of international renown who specializes in freedom of speech and who is a vocal critic of the right-wing party’s policies.

The case opened Wednesday in Warsaw’s Provincial Court against Wojciech Sadurski, a constitutional law professor at the universities of Warsaw and Sydney and a prominent public intellectual.

Law and Justice, the ruling party, is suing Sadurski for defamation over a 2018 tweet in which he called the party an “organized criminal group.”

Free speech organizations see the case as an attempt to silence and intimidate its opponents. They are calling for the case to be dropped.

Sadurski had called on Twitter this week for “all who care about the freedom of speech” to attend the proceedings against him.