THE HAGUE — A United Nations tribunal on Thursday acquitted a Serbian nationalist, Vojislav Seselj, of war crimes and of crimes against humanity for his role in the Balkan wars of the 1990s, igniting a celebration by his followers and outrage among relatives of the victims.

“It was the only possible verdict,” Mr. Seselj said in a defiant news conference in Belgrade, where he had gone for cancer treatment in 2014 and stayed on. “When I went to The Hague, I knew they could not prove any crime. There, I broke all the false accusations.”

To those representing the victims, the acquittal was as gut-wrenching as it was surprising. “This verdict is offensive and shocking, and the process of reconciliation among people and neighboring countries will be devastated,” said the Croatian foreign minister, Miro Kovac, in a statement. “All those in the world who incite war, calling for ethnic cleansing and a policy of forced changes to internationally recognized borders were given a moral blank check with this verdict.”

The acquittal by a three-judge panel came one week after the same court, the International Criminal Tribunal for the former Yugoslavia, convicted the former Bosnian Serb leader Radovan Karadzic of genocide and 10 other counts of grave crimes.