A world-renowned Turkish pianist is on trial on charges of inciting religious hatred and insulting Islam in comments he made on Twitter in April.

Fazil Say appeared in an Istanbul courtroom Thursday to defend himself against the charges, which stemmed from several tweets making jokes about Muslims. If convicted, he faces up to 18 months in prison.

Say has served as cultural ambassador for the European Union and has performed with several orchestras, including the New York Philharmonic and the Berlin Symphony Orchestra.

His case has raised concerns Turkey is tightening controls on freedom of speech. At least 100 demonstrators showed up at the courthouse to protest his trial.

Turkey has a secular government, but nearly all its population is Muslim.