The protesters lit 1,024 candles in memory of 1,024 Croatian soldiers and civilians still unaccounted-for from the war.
Party leaders Ruza Tomasic and Pero Kovacevic said that Serbian President Boris Tadic, who is expected to make an official visit to Zagreb next week, should tell the truth about the missing people.
Tomasic said that they "have come here to do what the Croatian government should have done long ago."
She accused Tadic of "cynicism and impudence" when he brought incomplete documentation to the hospital in Vukovar about the wounded and the missing during his recent visit to that eastern Croatian town.
Tomasic said that of 25 documents which Tadic brought back from Belgrade, only four were relevant.