Three weeks in to their devastating invasion, Russian forces also stood accused of bombing a theatre sheltering many civilians and marked with the word "children".
Kyiv emerged from a 35-hour curfew to its own fresh destruction, as Russian troops try to encircle the Ukrainian capital as part of their slow-moving offensive.
Beneath a Kyiv apartment block damaged by a downed rocket, AFP journalists saw a distraught man crouched over a body draped in a bloodstained cloth, after the latest in a series of early-morning attacks.
The 21 were killed when overnight artillery fire pounded a school and a cultural centre in the town of Merefa outside the hard-hit eastern city of Kharkiv, regional prosecutors said.
In besieged Mariupol to the south, searchers were combing through the smoking rubble of the Drama Theatre.
Ukrainian officials said more than 1,000 civilians had been sheltering in a basement bomb shelter beneath the theatre, and that Russian shelling was continuing. Human Rights Watch said it was at least 500.
'Tear down this wall'
President Volodymyr Zelensky said the "number of dead is not yet known" at the theatre, but the airborne attack showed "Russia has become a terrorist state".
Zelensky addressed the German parliament a day after a speech to the US Congress, when he secured $1 billion in new US military aid, including Stinger anti-aircraft missiles used against Soviet forces in Afghanistan.
Zelensky reached back to that Cold War era as he drew on a 1987 speech in Berlin by US president Ronald Reagan: "Dear Mr Scholz, tear down this wall," he implored German Chancellor Olaf Scholz.
"It's not a Berlin Wall -- it is a wall in central Europe between freedom and bondage and this wall is growing bigger with every bomb."
In an overnight video message, Zelensky also urged Russians to lay down their arms and abandon an invasion that has drawn swingeing Western sanctions against President Vladimir Putin's regime.
"If your war, the war against the Ukrainian people, continues, Russia's mothers will lose more children than in the Afghan and Chechen wars combined," he said, referencing the thousands lost in those conflicts.
Putin lashes 'traitors'
US President Joe Biden called Putin a "war criminal", triggering fury in the Kremlin, as the Russian leader also lashed out at "scum and traitors" at home who he said were undermining the war effort.
Russia's defence ministry denied it had targeted the theatre in Mariupol, where local officials say more than 2,000 people have died so far in indiscriminate Chechnya-style shelling.
The ministry said the building had been mined and blown up by members of Ukraine's far-right Azov Battalion, a claim dismissed in the West as Russian disinformation.
Satellite images of the theatre on March 14 shared by private satellite company Maxar showed the words "children" clearly etched out in the ground in Russian on either side of the building.
Officials posted a photo of the building, its middle part completely destroyed and thick white smoke rising from the ruins.
"The only word to describe what has happened today is genocide, genocide of our nation, our Ukrainian people," Mariupol mayor Vadim Boychenko said.
Addressing the Bundestag by video, Zelensky issued a strong rebuke of Germany's years-long reluctance to sever energy and business ties with Russia.
"We turned to you," he said. "We told you that Nord Stream (gas pipelines) was a kind of preparation for the war.
"And the answer we got was purely economic -- it is economy, economy, economy but that was the mortar for the new wall."
The broader economic consequences from the war could cut global growth by "over one percentage point" in the next 12 months, the Organisation for Economic Cooperation and Development said.
However, NATO members have resisted Zelensky's pleas for direct involvement through a no-fly zone over Ukraine, warning it could lead to World War III against nuclear-armed Russia.
Putin, at a televised government meeting Wednesday, insisted the invasion was "developing successfully", adding: "We will not allow Ukraine to serve as a springboard for aggressive actions against Russia."
He also condemned the Western sanctions as "economic blitzkrieg", after Russia was frozen out of much of the Western financial system.
However, the Russian finance ministry said Thursday it had made interest payments worth $117.2 million on two foreign bonds, avoiding a default for now.
From rackets to rifles
More than three million Ukrainians have fled across the border, mostly women and children, according to the UN.
With stop-start peace talks ongoing, officials in Kyiv said Russia had agreed to nine humanitarian corridors Thursday for fleeing refugees, including one out of Mariupol.
Kremlin spokesman Dmitry Peskov had said a "compromise" outcome in the talks would centre on Ukraine becoming a neutral state comparable to Sweden and Austria -- an idea roundly rejected by Kyiv.
Ukraine wants an immediate ceasefire, Russia to withdraw and the West to guarantee its security -- proposals that are equally anathema to Moscow.
French Foreign Minister Jean-Yves Le Drian told Le Parisien newspaper that Russia persisted with "maximalist demands -- wanting Ukraine to surrender and intensifying siege warfare".
Russia was reprising the brutal tactics it used in Chechnya and Syria, he said, and resorting to "talks with no objective other than pretending that they are negotiating".
Many Ukrainians agree that Russia's stated commitment to negotiations is contradicted by its tactics against their cities, and are zealous about defending them.
Retired tennis player Sergiy Stakhovsky, who knocked Roger Federer out of Wimbledon in 2013, has traded his racquet for a rifle and returned to Kyiv.
His wife was distraught at the decision, Stakhovsky told AFP as he patrolled the city in khaki camouflage, toting a Kalashnikov. But, he said, "I knew I had to go there."