Ivanovic was questioned then placed in custody "as an alleged suspect in an ongoing war crimes investigation, together with allegations of aggravated murder after the conflict" according to a statement from the European Union Rule of Law Mission (Eulex) yesterday.
Eulex is the EU's police and justice mission in the region.
Ivanovic, can be held in custody for up to 48 hours.
The 60-year-old had presented himself voluntarily for questioning in the northern flashpoint city of Kosovska Mitrovica, accompanied by his lawyer.
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But the 40,000 or so Serbs living in the north do not recognise its independence.
Kosovo has been recognised by more than 100 countries, including the United States and a majority of the EU states.
Ivanovic unsuccessfully ran for the post of mayor in north Mitrovica last December.