Before the curtains were lowered, the courtroom could be seen in a state of confusion, the BBC's Anna Holligan reports from The Hague.
An ambulance could later be seen arriving outside the tribunal while a helicopter hovered above the scene.
Several emergency rescue workers also rushed into the building carrying equipment in backpacks.
More than an hour after the incident, a court guard told Reuters news agency said Praljak was still "being treated".