The agreement to exchange 1000 prisoners each was the only concrete step towards peace to emerge last week from the first direct talks between the warring sides in more than three years, when they failed to agree a ceasefire.
Both sides said they had each released 270 soldiers and 120 civilians on Friday, with more due to be released on Saturday and Sunday.
The freed Russians are currently in Belarus, which neighbours Ukraine, receiving psychological and medical assistance before being moved to Russia for further care, the Russian Defence Ministry said.
They include civilians captured inside Russia's Kursk region during a Ukrainian incursion.
"Today, almost 400 people are home," Ukrainian President Volodymyr Zelenskiy said in a televised address that confirmed the plan for both sides to release 1000 in the coming days.
"We will definitely return everyone. Every one of our citizens."
The released Ukrainians arrived at a hospital in the northern Chernihiv region in buses and filed out, pale, most of them with shaven heads and wrapped in Ukrainian flags.
Referring to the prisoner swap earlier on Friday, US President Donald Trump, who had pressed the sides to meet last week, wrote on Truth Social: "Congratulations to both sides on this negotiation. This could lead to something big???"
Hundreds of thousands of soldiers on both sides are believed to have been wounded or killed in the war, although neither side publishes accurate casualty figures.
Tens of thousands of Ukrainian civilians have also died as Russian forces have besieged and bombarded Ukrainian cities.
Ukraine says it is ready for a 30-day ceasefire immediately.
Russia, which launched the war by invading its neighbour in 2022 and occupies about a fifth of Ukraine, says it will not pause its assaults until conditions are met first.
A member of the Ukrainian delegation called those conditions "non-starters".
Defence Minister Rustem Umerov told reporters at a hospital that the swap was "the first stage" and that Ukraine still hoped to secure a ceasefire.
"Ukraine wants the ceasefire. And we hope that the US will support Ukraine in achieving the ceasefire," he said.
Russia says it is ready for talks while the fighting goes on, and wants to discuss what it calls the war's "root causes," including its demands Ukraine cede more territory, and be disarmed and barred from the NATO military alliance.
Ukraine says that is tantamount to surrender and would leave it defenceless in the face of future Russian attacks.
Russian Foreign Minister Sergei Lavrov said on Friday his country would hand Ukraine a draft document outlining its conditions for a long-term peace agreement once the current prisoner exchange is completed.
12°C