One of the bodies of the men had his hands tied, and the corpses were strewn over several hundred metres (yards) of the residential road in the suburban town northwest of the capital.
The cause of death was not immediately clear, although at least one person had what appeared to be a large head wound.
Ukrainian officials said they did not yet know who the victims were or how they died.
“This territory was not under Ukrainian control” when they were killed, a Kyiv regional official told AFP on condition of anonymity.
“We just entered Bucha… we don’t yet know what went on there.
“It could be civilians killed by bombardment or they could have been shot by Russian soldiers. Law enforcement officials will deal with it.”
Ukraine on Saturday said Russian forces were making a “rapid retreat” from northern areas around the capital Kyiv, and declared earlier this week that Bucha had been “liberated”.
But the town had been left devastated by the fighting, with gaping holes from shell explosions in apartment blocks and crushed cars littering the streets, AFP journalists who accessed the town said.
Sixteen of the 20 corpses found on one street in Bucha were lying either on the pavement or by the verge. Three were sprawled in the middle of the road, and another lay on his side in the courtyard of a destroyed house.
An open Ukrainian passport lay on the ground next to the person who had his hands tied behind his back with a piece of white cloth. Two other people had white cloth tied around their upper arms.
All were wearing civilian clothes — winter coats, jackets or tracksuit tops, jeans or jogging bottoms, and trainers or boots.
Two of them lay tangled up in bicycles while another was next to an abandoned car. Some lay face up, with their limbs askew, while others were lying face down.
The skin on the faces of the dead people had taken on a pale waxy appearance, and they had dark nails, indicating that they may have been there for at least a number of days.
Signs of the violence were visible in the street in Bucha, which was littered with debris and downed power lines, with all the neighbouring the houses apparently deserted.
One silver car was covered with bullet holes, another had been partly crushed, while a burned out van lay near one group of bodies.
A line of around a dozen ambulances moved towards the scene on Saturday. Ukrainian authorities say they have been clearing bodies from the region for days.
Two other bodies were also seen in the town. One of them was covered by a sheet near a badly damaged apartment complex near Bucha’s railway station.
Bucha and nearby Irpin were the scene of some of the fiercest fighting since Russia invaded Ukraine on February 24, as Moscow’s forces pushed to encircle Kyiv.
The two leafy commuter towns held out but at a terrible price, with most residents fleeing relentless shelling and rocket attacks.
Ukrainian forces have only been able to move fully into Bucha in the last day or two, and the town has been inaccessible for nearly a month.
Troops in the town were handing out aid to residents for the first time since it returned to government control.
Russian forces now appear to be focusing attacks in the east and south of Ukraine, the government said.