Residential buildings, warehouses and a children's hospital were damaged in the attacks.
“The upper floors of a nine-storey residential building have collapsed and a fire has broken out,” the mayor of Kyiv wrote on the Telegram messaging service.
The ballistic missile attacks caught Kyiv residents by surprise. The missiles struck just three minutes after the air raid sirens sounded, meaning many did not have time to get to shelters.
"There used to be a building here. Now it's gone," said 45-year-old Maria Abramova. She had managed to get to her children's bedroom to wake them after the air raid siren went off. Then the rocket hit the block, shattering windows and shrapnel nearly killed her husband.
Day of mourning in Kyiv
A day of mourning for the dead is planned for Friday in Kyiv, with flags flying at half-mast and entertainment venues closed, according to Klitschko.
President Volodymyr Zelenskyy called on the outside world to act on Moscow's "escalation" of the full-scale war.
It was a massive attack - the Russians have long been preparing attacks with a combination of ballistic missiles, cruise missiles and drones, Zelensky said.
Of the 44 missiles fired, 39 were shot down by Ukrainian air defenses.
Want to pressure Moscow
Kyiv has been under attack almost every night recently, while Ukraine has intensified attacks on targets in Russia, partly to pressure Moscow into negotiations after nearly four and a half years of Russian invasion.
During the night of Thursday, around 250 drones were fired at the Russian capital, its mayor announced. Most of them were shot down.
The nighttime attacks prompted the Polish military to put fighter jets on alert. Similar measures were taken in Romania, where a drone crashed in an uninhabited area.
