One of the bodies of men was handcuffed, and the bodies were scattered across several hundred meters (meters) of a residential street in a suburban town northwest of the capital.
The cause of death was not immediately known, although at least one person had a major head wound.
Ukrainian officials said they still do not know who the victims were or how they died.
“This territory was not under Ukraine’s control” when they were killed, a Kyiv regional official told AFP on condition of anonymity.
“We just entered Bucha…we don’t know what happened there yet.
“It could be civilians killed by shelling or they could be shot by Russian soldiers. Law enforcement officials will deal with it.”
Ukraine on Saturday said Russian forces were in a “rapid retreat” from northern areas around the capital Kyiv, and announced earlier this week that Bucha had been “liberated”.
However, the town has been ravaged by the fighting, with holes left by shell explosions in apartment blocks and wrecked cars littering the streets, AFP journalists approached the town. town said.
16 of the 20 corpses were found on a street in Bucha, lying on a sidewalk or on the edge of a cliff. Three people were sprawled in the middle of the road, and another person was lying on his side in the yard of a destroyed house.
An open Ukrainian passport lay on the ground next to a man with his hands tied behind his back with a white cloth. Two others had white cloth tied around their arms.
All are dressed in civilian clothing – winter coats, jackets or sweatshirts, jeans or joggers, sneakers or boots.
Two of them lay cluttered on bicycles while another was beside an abandoned car. Some lie on their backs with their limbs bent, while others lie face down.
The facial skin of the dead had a pale waxy appearance, and they had dark fingernails, suggesting they might have been there for at least a few days.
Signs of violence can be seen on the streets of Bucha, which is littered with debris and power lines have collapsed, with all neighboring houses seemingly deserted.
One silver car was full of bullet holes, another was partially crushed, while a burned truck lay near a group of bodies.
A dozen ambulances rushed to the scene on Saturday. Ukrainian authorities said they had been removing bodies from the area for several days.
Two other bodies were also seen in the town. One of them was covered by a corrugated iron sheet near a badly damaged apartment block near Bucha’s train station.
Nearby Bucha and Irpin have been the site of some of the heaviest fighting since Russia invaded Ukraine on February 24, such as MoscowHis forces were pushed to encircle Kyiv.
Two leafy towns did exist, but at a terrible cost, with most of the inhabitants fleeing incessant shelling and missile attacks.
Ukrainian forces have only been able to fully move into Bucha in the past day or two, and the town has been inaccessible for almost a month.
Soldiers in the town are giving aid to the residents for the first time since it returned to government control.
The government says Russian forces are now concentrating their attacks in eastern and southern Ukraine.
