Andrzej Poczobut, a journalist and leader of the ethnic Polish minority in Belarus, went on trial Tuesday on charges of slandering and insulting Belarus' authoritarian president.
The 38-year-old Poczobut, who writes for the leading Polish newspaper Gazeta Wyborcza, faces up to four years in prison if convicted.
President Alexander Lukashenko has been cracking down on independent journalists and opposition activists since extending his rule through a controversial election last year that sparked mass street protests.
The arrest of Poczobut in April drew outrage in neighboring Poland. Polish Prime Minister Donald Tusk said Tuesday that Poland, which assumes the European Union presidency on July 1, will use all available EU instruments to make sure human rights are respected in Belarus.
"We will definitely never fail to support what is civic and democratic," Tusk said in Warsaw. "Everyone can count on us. But the efficiency of such activity, due to actions taken by the Lukashenko regime, is not satisfactory."
About 100 of Poczobut's supporters gathered outside the courthouse in the western city of Grodno, where his trial began Tuesday behind closed doors. Among them was his wife, Oksana, who was not allowed to enter.
As police escorted Poczobut into the courthouse, he held up two fingers in a "V," a sign for victory that is reminiscent of Poles' fight for democracy and independence in the 1980s.
A verdict is expected by the end of the week, according to defense lawyer Alexander Birilov.
On the Polish side of the nearby border, huge billboards with Poczobut's picture have been put up facing Belarus and calling on Lukashenko to set him free.
Gazeta Wyborcza ran a picture on its front page of Poczobut with tape across his mouth. The picture is being used in an Amnesty International campaign in defense of freedom of speech in Belarus.
Poczobut is facing charges not only for his stories in the Polish newspaper but also for what he wrote on a Belarussian opposition website and in his own blog.
Yuras Karmanau in Minsk, Belarus, and Monika Scislowska in Warsaw, Poland, contributed to this report.