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Mr Zelensky said: 'This is genocide. The elimination of the whole nation and the people.'

Mr Klitschko said pictures of the devastation were 'painful to see'. He added: 'They are killing civilians, they kill children, women.'

Fears of a genocidal plot grew as it was claimed Russia adopted a 'state technical standard for digging and maintaining mass graves' weeks before the invasion.

Troops were taught how to build the huge burial grounds for 1,000 corpses in just three days in a policy that came into effect on February 1 – three weeks before they marched on Ukraine.

Observers fear it could suggest Putin had 'planned genocide' in Ukraine on a scale 'unseen since World War Two'.

Russia pulled out of the cities of Kyiv and Chernihiv on Friday, retreating up to 25 miles in places and losing more than 30 towns in a significant victory for Ukraine.

But as the survivors emerged from their hiding places underground, their stories and the untold destruction surrounding them laid bare the horrors of the Russian occupation.

Between mangled tanks and ruined houses, bodies lined the streets of Bucha, a suburban town of 28,000 residents before the war.

Pictures too distressing to publish showed the body of one man bound and discarded in a sewer.

Others were strewn in front of a railway station and by the side of the road. On one street alone 20 bodies, all in civilian clothes, had their hands bound behind their back. A 14-year-old boy was among them.

Another photo showed three men, arms tied, sprawled on a roadside next to a pile of wooden pallets.

Others who appeared to have been fleeing were beheaded, according to the Telegraph.

Bucha's mayor, Anatoly Fedoruk, said: 'All these people were shot, killed, in the back of the head.'

He claimed 'entire families... children, women, grandmothers, men' were murdered trying to escape.

In nearby Irpin, another commuter town vacated by Russia last week, more bodies littered the parks. 'They shot girls and women, and then ran over them in a tank,' said its mayor Alexander Markushyn.

The historical echoes have not been lost on Ukraine and other post-Soviet nations.

Estonian prime minister Kaja Kallas said: 'Photos of murdered civilians in Irpin and Bucha by Russian troops recall the mass killing by Soviet and Nazi regimes. This is not a battlefield, it's a crime scene.'

Military experts said the mass killings were consistent with Russian practices during the second Chechen war in 1999-2000.

Jack Watling, a research fellow at the Royal United Services Institute, said: 'This was the plan, it was premeditated.

'If the Russian military had been more successful there would have been more towns like it.' But Russia dismissed the pictures of atrocities as 'fake'.

US Secretary of State Antony Blinken described evidence of Russian war crimes as 'a punch to the gut'.

The atrocities are likely to stiffen Western resolve to ensure Ukraine defeats Russia on the battlefield as the war enters a new phase.

Conservative MP and former Army officer Johnny Mercer said the war crimes were 'the end of the line' for those opposed to increasing military support. For the first time, Ukraine is set to receive tanks, which it will need to displace Russian forces in Donbas, the eastern area occupied by pro-Russian separatists.

Poland wants to provide T-72 tanks, which the Ukrainians already know how to use. Other Nato members are expected to 'backfill' tanks to Poland.