In memory of 42 soldiers and police killed in the operation, wreaths were laid at the Central Cross in Okucani by Prime Minister Ivo Sanader, President Stjepan Mesic, Parliament Speaker Vladimir Seks, War Veterans Minister Jadranka Kosor, Defence Minister Berislav Roncevic, and a state secretary at the Interior Ministry, Ivica Buconjic.
The state delegation will also attend mass for soldiers and civilians killed in the 1990s war of independence to be celebrated by military ordinary Juraj Jezerinac, and attend a display of army and police exercises at Okucani's soccer field.
At 1700 hours, PM Sanader will inaugurate a square in the town.
Operation Flash was launched on the morning of May 1, 1995 and was completed the next day. During the 31 hours, Croatian forces managed to retake 500 square kilometres of land from rebel Serb forces and regain control of the Zagreb-Lipovac highway.
About 7,200 Croatian soldiers and police officers were involved in the lightning offensive, 42 of whom were killed and 162 wounded.
The operation enabled the start of reconstruction of the Western Slavonia region and return of people displaced by war.
In retaliation for the defeat, Serb insurgents fired missiles with cluster munitions on Zagreb on May 2 and 3, 1995, killing six people and wounding 175, of whom 39 seriously.
The Hague-based International Criminal Tribunal for the former Yugoslavia (ICTY) later indicted Serb rebel leader Milan Martic for ordering the attacks.