A man said to strongly resemble fugitive Bosnian war crimes suspect Ratko Mladic is undergoing identity tests after being arrested in Serbia.
Respected Serbian radio station B92 said: "B92 has been able to verify that a man suspected to be Mladic is in custody."
Gen Mladic is the most prominent Bosnian war crimes suspect who is still at large.
Serbian President Boris Tadic is due to make a statement shortly.
Gen Mladic, the former Bosnian Serb military leader, is wanted by the Hague Tribunal on genocide and war crimes charges.
An interior ministry official confirmed for Reuters news agency that a suspect was being held.
"He has some physical features of Mladic, we are analysing his DNA now," the official said on condition of anonymity.
The man was arrested in Serbia on an anonymous tip-off, he added.
The report on B92 said the suspect had been using the name Milorad Komadic.
Serbia's war crimes prosecutors refused to confirm or deny that Gen Mladic had been detained, the Associated Press news agency reports.