Belarusian authorities detained a dissident activist on Sunday after diverting a jetliner he was on and forcing it to land in the capital, Minsk.
Media critical of President Alexander Lukashenko reported that Roman Protasevich was detained at an airport in Minsk.
The Ryanair plane was flying from Greece to Lithuania.
Belarus's state-run media say the authorities instructed the aircraft to make an emergency landing in Minsk, citing a bomb threat. But no explosives have been found on the plane.
Belarusian opposition leader Svetlana Tikhanovskaya condemned Lukashenko's government, saying, "It is obvious that this is an operation by the special services to hijack the plane in order to apprehend the activist."
European Council President Charles Michel and the International Civil Aviation Organization, or ICAO, have also expressed grave concerns on Twitter.
Mass protests broke out in Belarus after Lukashenko won a sixth term in last August's election.
Protasevich fled Belarus into exile like other opposition figures and founded a media outlet critical of Lukashenko's government.