The annual service, usually held in Austria at the site of the Croatian regime's last stand, was moved to Sarajevo because of coronavirus restrictions in Austria.

Croatian groups gather each year for the commemorations, co-organised by the Catholic Church in Bosnia and Croatia, often brandishing Nazi memorabilia.

The Nazi-hunting Simon Wiesenthal Center urged the Bosnian government to ban the Mass, labelling it a "travesty of memory and justice".

The Mass provoked outrage from most political parties, Jewish leaders the Serbian Orthodox Church and anti-fascist NGOs, who will hold a march during the Mass in central Sarajevo.

"It is wrong that religious ceremonies be held for those who have committed horrible crimes in this city," Nijaz Skenderagic of the association of anti-fascists said.

Sarajevo Archbishop Vinko Puljic, who will hold the Mass in the Bosnian capital's cathedral, rejected the accusations and said praying for victims' souls did not mean approval of their acts.

Since Saturday morning, police used metal fences to seal all roads leading to the cathedral for the service, due shortly after noon (1000 GMT).

Only 20 people were likely to attend the service, according to the archdiocese.

Croatia's WWII pro-Nazi regime persecuted and killed hundreds of thousands of Serbs, Jews and anti-fascist Croatians.

At the time, the so-called Independent State of Croatia (NDH) included Bosnia and parts of Serbia.

More than 10,000 Sarajevo residents -- mostly Jews but also Serbs and Roma -- were killed by the Ustasha regime during WWII.

Near the end of the war, the Ustasha, accompanied by civilians and Slovenian and Serbian collaborators, started to flee towards Austria hoping to surrender to British forces.

But the British refused their surrender.

The Ustasha were caught and executed by Josip Broz Tito's partisans in the border region between Austria and Slovenia.

The number of those killed in the Bleiburg area -- where the commemorations are usually held -- is still debated, but independent historians put it at tens of thousands.

Short memorial ceremonies for the victims are due also in Croatia later Saturday.