BELGRADE, Serbia—A judge finished interrogating former Bosnian Serb President Radovan Karadzic early Tuesday, triggering a procedure to hand over the accused mastermind of Europe’s worst massacre since World War II to a U.N. war crimes tribunal.

Karadzic, a psychiatrist turned die-hard Serbian nationalist politician, was arrested by Serbian forces and taken before the country’s war crimes court on Monday, indicating imminent extradition to the U.N. war crimes tribunal in The Hague, Netherlands.

“The questioning is over,” investigating judge Milan Dilparic said Tuesday, referring to the first step in a legal process that includes presenting Karadzic with the indictment and allowing three days for him to appeal any decision to extradite him.

While Serbia braced for a possible reaction from ultra-nationalists who are believed to have helped shelter Karadzic over the years, Bosnian Muslims jammed the streets of Sarajevo early Tuesday in celebration of the arrest.

Karadzic is charged with masterminding the deadly siege of Sarajevo and mass killings that the U.N. war crimes tribunal described as “scenes from hell, written on the darkest pages of human history.” They include the 1995 massacre of 8,000 Muslims in Srebrenica, Europe’s worst slaughter since World War II.