Veselin Vlahovic is the subject of three international arrest warrants
Spanish police say they have arrested a Montenegrin man accused of committing war crimes while commanding Serb forces during the 1992-95 Bosnian civil war.
Veselin Vlahovic was detained in the tourist resort of Altea, in Alicante.
Mr Vlahovic is known as the "Monster of Grbavica", a reference to the suburb of Sarajevo where he is alleged to have murdered, raped and tortured civilians.
He was imprisoned for three years in Montenegro for robbery and violence, but escaped in 2001 and vanished.
Mr Vlahovic is currently the subject of three international arrest warrants, and is also wanted in Spain for armed robbery.
Prosecutors in Bosnia have accused him of more than 50 cases of war crimes against the non-Serb population at the outbreak of the war.
"Vlahovic was a paramilitary and is suspected of murders, serious abuse, looting," spokesman Boris Grubesic told the AFP news agency.
Mr Vlahovic has also been sentenced to 15 years in jail in absentia for shooting dead a man in Serbia days after escaping from prison.