WARSAW, Poland (AP) — Poland’s ruling party is suing a law professor of international renown for defamation over comments he made on Twitter.

The case against Wojciech Sadurski, a constitutional law professor who is a vocal critic of Poland’s right-wing government, opened Wednesday in Warsaw’s Provincial Court.

Sadurski, who has positions at the universities of Warsaw and Sydney and specializes in freedom of speech, frequently takes to Twitter to make critical comments of the Law and Justice party.

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

Sadurski testified before the one-judge panel to say he only expressed his personal opinion and acted, in his mind, in Poland’s interest.

Free speech organizations see the case as an attempt by the ruling party to silence and intimidate its opponents and 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.

The tiny courtroom was filled with some two dozen reporters and members of the opposition and human rights organizations’ members.

The court is set to deliver its verdict Dec.16.