BELGRADE — A Serbian guerrilla leader in Yugoslavia's breakaway republic of Croatia has been imprisoned by Muslims in the neighboring republic of Bosnia-Herzegovina, rebel sources said on Monday.The sources in Knin told Reuters by telephone that Milan Martic, police chief in the Serbian-controlled Krajina region, was detained by Muslims Sunday in the town of Bosanska Krupa. He was arrested when his car broke down in the Muslim-populated village of Otoka, the sources said. No charges were pressed but angry Muslims surrounded the police station where he was held to prevent his release. Martic is a key figure in the rebellion by the 600,000-member Serbian minority in Croatia against the republic's moves to secede from Yugoslavia. Belgrade radio said local Serbs in the area were moving on Bosanska Krupa to free Martic. The Bosnian government was holding an emergency session fearing inter-ethnic conflicts over Martic's arrest, the radio said. The trouble in Croatia has been spreading into neighboring Bosnia, whose population is a mix of Muslims, Serbs, and Croats. Bosnia is ruled by a fragile coalition of three nationalist parties.