Ukrainian President Volodymyr Zelenskiy announced the prisoner release in his nightly address on September 21.
Zelenskiy said five military commanders, including leaders of the defense of Azovstal earlier this year, were taken to Turkey as part of an operation prepared in advance and agreed with Turkish President Recep Tayyip Erdogan.
The released prisoners will remain in Turkey "in total security and in comfortable conditions" until the end of the war, Zelenskiy added.
Among the 55 prisoners turned over to Russia is Viktor Medvedchuk, a former Ukrainian lawmaker and ally of Russian President Vladimir Putin, Zelenskiy said. Medvedchuk had been held under house arrest since May of 2021 on charges of treason and financing terrorism.
Zelenskiy had previously proposed swapping Medvedchuk for prisoners of war being held by Russian forces.
The swap is the biggest exchange between the two sides since the start of Russia's invasion in February.
"We have managed to liberate 215 people," the Ukrainian presidency's chief of staff Andriy Yermak announced on Ukrainian television. Yermak said 108 of them were fighters of the Azov Regiment.
Yermak said the exchange of prisoners took place in several stages and in different locations.
Ten prisoners of war including five Britons and two Americans, who were transferred from Russia to Saudi Arabia earlier, were part of the swap, Zelenskiy said.
Erdogan told U.S. broadcaster PBS on September 19 that Russia and Ukraine had agreed to swap 200 prisoners but gave no details.
Mariupol, a port city on the Sea of Azov in southeastern Ukraine, withstood weeks of relentless Russian bombardment as resistance fighters and some civilians remained in a network of underground tunnels at the Azovstal steel plant.
Ukrainian forces, including members of the Azov Regiment, in May ended the weekslong siege by agreeing to surrender.
Around 2,500 combatants were taken captive by Moscow's forces after calling a halt to their resistance. Some of the prisoners were part of an exchange that took place in June. Of the 144 freed then, 95 were Azovstal defenders.
The Azov Regiment is a former volunteer battalion that has drawn controversy for its links to far-right figures.
Comment: Interesting way of putting it.
An attack on a prison complex in July in an area of the Donetsk region in eastern Ukraine that is under Kremlin-backed separatist control killed dozens of Ukrainian prisoners of war, reportedly including members of the Azov Regiment.
Comment: Saudi Crown Prince MBS mediated the release of the 10 foreign combatants, which in addition to those mentioned above, included a Croatian, Moroccan, and Swede. The five Brits (Aiden Aslin, Shaun Pinner, John Harding, Dylan Healy, and Andrew Hill) are now back in the UK.
Apparently, the captive Russian pilots were not among the first batch returned to Russia. Here's how a Russian source describes the problem:
Kadyrov isn't happy either: