Reuters witnessed buses leave the steelworks and five of them arrive in the Russian-held town of Novoazovsk on Tuesday, where Moscow said they would be treated for wounds.
In one, marked with the Latin letter 'Z' that has become the symbol of Russia's assault, wounded men were lying on stretchers three bunks high. One man was wheeled out, his head tightly wrapped in thick bandages.
While both sides spoke of a deal under which all Ukrainian troops would abandon the huge steelworks, many details were not yet public, including how many fighters remained inside and whether any form of prisoner swap had been agreed in advance.
The Kremlin said Russian President Vladimir Putin had personally guaranteed the prisoners would be treated according to international standards.
"The Mariupol garrison has fulfilled its combat mission," the General Staff of Ukraine's Armed Forces said in a statement.
"The supreme military command ordered the commanders of the units stationed at Azovstal to save the lives of the personnel ... Defenders of Mariupol are the heroes of our time."
Russian defence ministry video showed fighters leaving the plant in daylight, some carried on stretchers, others with hands up to be searched by Russian troops.
Russia said at least 256 Ukrainian fighters had "laid down their arms and surrendered", including 51 severely wounded. Ukraine said 264 soldiers, including 53 wounded, had left the plant and efforts were under way to bring out the rest.
The surrender appears to mark the end of the battle of Mariupol, where Ukraine believes tens of thousands of people were killed under months of Russian bombardment and siege.
The city now lies in ruins. Its complete capture is Russia's biggest victory of the war, giving Moscow total control of the coast of the Sea of Azov and an unbroken stretch of eastern and southern Ukraine about the size of Greece.
But it comes as Russia's campaign has faltered elsewhere, with its troops around the city of Kharkiv in the northeast lately retreating at the fastest pace since they were driven out of the north and the area around the capital Kyiv at the end of March.
Authorities on both sides gave few clues about the ultimate fate of Mariupol's last defenders. Ukraine's Deputy Prime Minister Iryna Vereshchuk said Kyiv aimed to arrange a prisoner swap for the wounded Ukrainians once their condition stabilises, but neither side disclosed terms for any specific deal.
"We hope that we will be able to save the lives of our guys," Ukrainian President Volodymyr Zelenskiy said. "There are severely wounded ones among them. They're receiving care. Ukraine needs Ukrainian heroes alive."
The United Nations, which had helped civilians evacuate from Azovstal this month, was not operationally involved in the evacuation, UN spokesperson Saviano Abreu said.
Mariupol is the biggest city Russia has captured since its Feb. 24 invasion, giving Moscow a clear-cut victory for the first time in months, during which its campaign in Ukraine has mostly faced military disaster against an underestimated foe.
The United Nations and Red Cross say thousands of civilians died under Russia's siege of the once prosperous port of 400,000 people, with the true toll uncounted but certain to be Europe's worst at least since the 1990s wars in Chechnya and the Balkans.
For months, Mariupol's residents were driven into cellars under perpetual bombardment, with no access to food, fresh water or heat, and bodies littering the streets. Two strikes - on a maternity ward and a theatre where hundreds of people were sheltering - became worldwide emblems of Russia's tactic of devastating population centres.
Thousands of civilians are believed to have been buried in mass graves or makeshift pits in gardens. Ukraine says Moscow sent mobile cremation trucks to erase evidence of civilian deaths, and forcibly deported thousands of residents to Russia.
Moscow denies targeting civilians or deporting them, and says it has taken in refugees. It says it is now restoring normal life to the city, part of the Donbas region it claims on behalf of separatists it has backed since 2014.
