Nato fighter jets have been scrambled, while Russian missiles are believed to have entered the bloc's airspace, amid a "massive" Russian attack on Ukraine.
Volodymyr Zelensky issued an urgent appeal to his allies for help as he revealed his air force had alerted him to a "high probability" of Vladimir Putin ordering the strike during the early hours of Thursday morning.
Writing on social media, he stressed: "It is important that our partners fully understand what is happening and that the protection of lives directly depends on their readiness, or lack thereof, to help us with air defence missiles.
"First and foremost, this concerns the United States and those partners that have Patriot missiles and can provide them to support our defence.
"Every partner knows what is needed and fully understands the consequences of marking time."
Then, as Thursday morning broke, Kyiv Mayor Vitali Klitschko confirmed the Ukrainian capital had come under attack from Russian ballistic missiles.
In a Telegram post, the boxer-turned-politician urged residents to seek shelter as a handful of non-residential buildings in Kyiv caught fire.
Multiple explosions could be heard across the city as the assault from the skies unfolded, one witness said.
Six people - including children - were killed in Kryvyi Rih, a city in central Ukraine, with eight people injured. One was killed in Kyiv, while 15 more were injured in Lviv with many more beneath rubble.
The Ukrainian President revealed that his air force had alerted him to a 'high probability' of Russian strikes on Thursday
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Flames and smoke were seen pouring out into the sky over Kyiv amid the attack
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In neighbouring Poland - a Nato member - air raid sirens sounded across towns in Lublin, close to the Ukrainian border, after authorities detected an "airstrike threat".
One British tourist, Ryan, shared footage of the sirens on social media in the early hours of the morning, saying the sound "scared the hell out of" him.
"This information comes directly from the Government Security Centre. It is credible and confirmed," a spokesman for the Lublin Voivode said.
The Polish Armed Forces confirmed that Russian cruise missiles had crossed into Polish airspace as they bombarded Ukraine.
In a statement, the Operational Command of the Polish Armed Forces said it had "activated the necessary forces and resources at its disposal" to respond to the threat.
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Multiple explosions could be heard across the city as the assault from the skies unfolded, one witness said
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"Fighter aircraft and an early warning aircraft have commenced operations, and ground-based air defense and radar reconnaissance systems have reached a state of readiness," they added.
Mr Zelensky's pleas to allies follow his visit to the White House on Tuesday, where he sat down with Donald Trump in the Oval Office to discuss licensing arrangements for the production of Patriot interceptors.
Only Patriot systems are capable of shooting down Russian missiles - but Ukraine is short of PAC-3 interceptor missiles.
The Ukrainian President said he had held "a good meeting" with his American counterpart.
The Ukrainian President said he had held 'a good meeting' with his American counterpart on Tuesday
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After the talks, Mr Trump posted photographs of the pair on social media, alongside the caption: “Many things were discussed. The meeting went very well!”
The Ukrainian leader said he is now counting on “practical results” following Tuesday's meeting in Washington.
Relations between Mr Zelensky and the US President have improved since the infamous flare-up in the Oval Office at the beginning of his second term.
Mr Trump had pledged the previous month to grant Ukraine a licence to manufacture Patriot air defence systems domestically.