BANJA LUKA, Bosnia and Herzegovina — After his 21-year-old son David was found dead in a creek last March, Davor Dragicevic, a waiter in a cafe, took matters into his own hands.

Dissatisfied with official explanations that David Dragicevic had been a drug addict and a thief, and had killed himself or been murdered by a criminal gang, Mr. Dragicevic started a one-man protest movement that has grown into the largest antigovernment demonstration in Bosnia in decades.

Now the elder Mr. Dragicevic, 49, has not been seen in public since Dec. 30, when he led thousands on a march around Banja Luka, the capital of the Serb autonomous region in eastern Bosnia, demanding the resignation of the interior minister, Dragan Lukac. He was last seen by local journalists as he was being pursued by plainclothes police officers.

The Bosnian Serb police have issued an arrest warrant for Mr. Dragicevic, charging him with incitement and threatening public safety. On Monday, Mr. Dragicevic posted a video message on Facebook.