Ihor Talalay was helping evacuate citizens from the Ukrainian city of Mariupol in March when Russian forces captured him, marking the start of a harrowing three-months ordeal for the 25-year-old.
"It was extremely difficult. What kept me going was the thought that I want to see justice and victory over those who are doing these terrible things to me," Talalay said, adding some of what he experienced was too painful to recall.
Speaking with several others at the Vienna-based Organization for Security and Co-operation in Europe (OSCE) on Wednesday, Talalay said he was moved several times through various detention facilities in the Donetsk region.
The worst part was when he was kept in a three-by-three-metre (9.8 foot) cell with around 30 others, all standing up. Some would return to the cell with torture marks from electricity.
He said besides beatings, he and others suffered "constant hunger", being given only a handful of porridge twice a day and a watery soup.
Earlier this month, an OSCE report expressed "grave concern" about alleged mistreatment of tens of thousands of Ukrainians at so-called filtration centres set up by Russia in Ukraine.
The report described "harsh interrogations and humiliating body searches in such centres".
It added those found to have collaborated with Kyiv "often simply disappear" with some allegedly transferred to Russian-controlled territories, where they are detained or even murdered.
Media Initiative for Human Rights (MIHR), a Kyiv-based Ukrainian NGO, said on Wednesday that it had identified at least 18 such filtration centres.
Whereas "filtration processes" were in place in insurgency-wrecked eastern Ukraine even before Russia's invasion of Ukraine on February 24, they have become "systematised" and are being carried out "on a very huge, overwhelming scale," according to Stanislav Miroshnychenko, a Ukrainian journalist who now works at MIHR.
Another witness, Yurii Berezovskyi, said he lived in constant fear in Luhansk after Russian forces occupied the region. He was detained and questioned before deciding to try to flee via Russia.
"I was lucky because I was let go. But (the Russians) said 'We can come back'... The most terrible and scary part in all of this is you don't know where you'll end up," the 32-year-old former music teacher said.
Olha Tabachuk, also speaking to reporters after addressing a meeting of OSCE delegates, said her family had not heard from her son, 38, since he was detained while helping Ukrainians evacuate.
"I don't know whether he is alive or not... It's pure terror... I can't believe this is happening in the modern day," the 62-year-old said.
The OSCE, the world's largest security body founded in 1975, currently has 57 member states, including Russia.