Pro-Russian separatist leader Valery Bolotov was detained by Ukrainian border guards on Saturday then freed by armed supporters in a firefight at the checkpoint, a Ukrainian spokesman said.
The incident took place just a few hours before peace talks in the eastern town of Kharkiv. The talks were initiated by Ukrainian authorities to ease the situation in east.
Bolotov, the self-proclaimed governor of Ukraine's eastern Luhansk region, was detained at the checkpoint as he returned from treatment in Russia for wounds he suffered in an attack on Tuesday, border guards spokesman Oleh Slobodyan said.
About 200 armed separatists went to the checkpoint and demanded his release. A fight broke out in which the separatists used grenades and machine guns.
"They recaptured Bolotov. We are trying to find out whether there are victims," Slobodyan said.
The eastern rebellion by pro-Russian separatists has shaken Ukraine and pushed Western powers into a confrontation with Russian President Vladimir Putin, whom they accuse of destabilising Ukraine.