Vekic told a news conference in Osijek that this would be an "all-Croat gathering" at which participants would voice their opposition to any further attempts to criminalise the war of independence.
The aim of the rally is to show that putting Croatian war veterans behind bars for alleged was crimes 20 years after the war is unacceptable, Vekic said.
He added that they would show solidarity with Djuro Brodarac, Vladimir Milankovic and Drago Bosnjak, who are suspected of war crimes in Sisak.
HS official Vencel Culjak said that at the rally the party would also express its opposition to plans of Croatia joining the European Union.