The media reported that the candidate of the coalition Slate for Sandzak, Ruzdija Durovic, was killed and his cousin Sulejman Durovic gravely wounded in a shooting in the capital of the majority Muslim Sandzak region.
The police cordoned off the crime scene and no official statement was given of the circumstances of the incident.
In Serbia's parliament, the four parties making up Slate for Sandzak support the Vojislav Kostunica cabinet.
Citing unofficial sources, Belgrade's Radio B92 said the incident was preceded first by bickering and then a fight between sympathisers and members of the two strongest regional parties, Sulejman Ugljanin's Party of Democratic Action and Rasim Ljajic's Sandzak Democratic Party.
Radio B92 also said the killers had been identified but were on the run.
Early local elections were held in four Sandzak municipalities after Serbia's government last spring dissolved local assemblies and introduced temporary measures.