"We are hoping for the day when we will be able to say Sandor Kepiro is guilty. He is guilty," Aleksandar Veljic of the Association for the Remembrance of Holocaust told the rally in Novi Sad 80 kilometers (50 miles) north of Belgrade.
A Hungarian court last week cleared Kepiro, 97, of involvement in the January 1942 Novi Sad massacre of more than 1,000 civilians, mainly Serbs and Jews, due to lack of evidence. The verdict is subject to appeal.
"This verdict was a disgraceful decision, shameful for survivors, victims and mankind," Veljic said.
Kepiro, a Hungarian national, served as a gendarme during the war, when parts of Serbia were occupied by troops from Hungary, then allied with Nazi Germany. The Novi Sad massacre was ordered in retaliation for attacks by communist partisans.
Kepiro was accused of being involved in a series of events in which people were rounded up and sent to their deaths before a firing squad or were pushed under the ice of the Danube river.
'Rounded up people and killed them all'
Kepiro was also charged with being a member of a squad that murdered people in their homes.
"No one knew what they were preparing, they just rounded up peaceful people from their homes and killed them all," said Ljubisa Lekic, 87, who lost her father and two uncles in the atrocities.
Kepiro lived in Argentina from 1948 to 1996. He was spotted in 2006 in Budapest by the Nazi-hunting Simon Wiesenthal Center, which informed Hungarian authorities.
As many as 1.7 million people died in former Yugoslavia during World War II, mainly Serbs, Jews and Roma, but also other nationalities, many in death camps.
Serbian authorities, including the country's top war crimes prosecutor, Vladimir Vukcevic, who helped bring the case against Kepiro in 2008, said they were disappointed by the verdict.
Belgrade has also sought to try Kepiro for war crimes, but his extradition was put on hold due to the trial in Hungary.
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