Captured Ukrainian fighters face being paraded in public prison cages on stage at Mariupol’s grand Philharmonic hall during an anticipated "show trial" of the prisoners of war.
The trial of the Azov prisoners could happen as soon as August 24, Ukraine’s independence day, to give the Russians an appearance of a victory following months of failures in the eastern Donbas region.
The makeshift holding cages and planned open trial heighten the risk of public executions, which Russian forces are said to be planning to shatter Ukrainian morale.
Denis Pushilin, head of the self-proclaimed Donetsk People's Republic, recently confirmed the proceedings would happen before the end of the summer.
Moscow detained more than 2,500 fighters who spent months holed up in Mariupol Azovstal steel works during a months-long siege of the southern port city.
Having surrendered in May, the Ukrainian troops - hundreds wounded and injured - limped out of the vast industrial complex after being told to give up their position by Kyiv.
"All prisoners of war, and in particular the 'Azovstal' defenders are combatants who legally defended their country," Mykhailo Podolyak, a senior aide to the Ukrainian president, wrote on Twitter.
He added that the "Mariupol cages" are "an official war crime of Russia", calling on international humanitarian organisations to intervene.
Vadym Boichenko, the mayor of Mariupol, said: "They want to hold this mayhem at the Philharmonic Hall in Mariupol somewhere around independence day.
"Russia has no success in the battlefield, so it wants to show the 'victory' over the prisoners of war.
"This is what is called a war crime. We had an arrangement that the soldiers who surrendered would return back to Ukraine."
Mr Pushilin said the trials in Mariupol would be open to the media and international representatives.
"The task is to hold the most open trial possible, so that no-one says any doubts," he added.
Some 10,000 captured Ukrainians, including military personnel and civilians, are being held captive in Russian prisons, according to Ukrainian estimates.
Two British soldiers – Aiden Aslin, 28, and Shaun Pinner, 48 – fighting for the Ukrainian army were sentenced to death in June by pro-Russian rebels in eastern Ukraine.