An armed man freed a senior policeman after taking him hostage in the central Ukrainian city of Poltava on Thursday.
He had driven off with him before abandoning both the car and the officer and running into a forest, the police said.
Authorities were trying to arrest a man suspected of car theft when he took out a grenade and threatened to kill one of the officers, Deputy Interior Minister Anton Gerashchenko said on Facebook.
After negotiations, the man exchanged the police officer for a police colonel and drove off with him in a car provided by police.
Gerashechenko and local authorities say police allowed the man to leave the town "for the safety of citizens".
Police said that the intruder left the car in the middle of the road and ran to the forest.
And that the hostage was released unharmed, while an operation to detain what they describe as 'a dangerous armed criminal' continues.
It was the second hostage-taking in Ukraine in three days.
On Tuesday, an armed man held 13 people captive on a bus for hours before releasing them.