Loud explosions shook the Ukrainian capital for hours during the night, with many people sheltering in subway stations after authorities issued air raid warnings.
Emergency crews were still digging through the rubble of collapsed and charred apartment buildings in search of victims as dawn broke on Thursday.
Russia's defence ministry said the deadly bombardment was in response to Ukraine's long-range strikes that have caused severe fuel shortages and put pressure on President Vladimir Putin.
Ukraine's increasingly frequent and large-scale attacks - described by President Volodymyr Zelenskiy as a 40-day blitz - have especially targeted oil refineries, causing a fuel crisis that has frustrated Russians, more than four years after Moscow's full-scale invasion of its neighbour.
Ukrainian officials say they are trying to force Putin to the negotiating table, but so far Moscow's response has been to hit back.
Diplomatic efforts to end the war, most recently by the Trump administration, have not produced results.
The attack killed 20 people in Kyiv, according to Kyiv city administration head Tymur Tkachenko.
More than 90 others were injured, according to Kyiv mayor Vitali Klitschko.
Foreign Minister Andrii Sybiha said it was a "night of horror" in the capital.
Damage was recorded in 30 locations across the city, mainly residential buildings and civilian infrastructure, according to Tymur Tkachenko, head of the Kyiv City Military Administration.
Some 20 residential buildings were damaged, Interior Minister Ihor Klymenko said.
In Kyiv's Desnianskyi district, people were trapped inside a damaged nine-storey residential building, and in the Darnytskyi district six levels of a nine-storey building collapsed.
Russia's General Staff chief General Valery Gerasimov reported the results of the "massive retaliatory strike" to Putin, Kremlin spokesman Dmitry Peskov said.
The bombardment was "exclusively against military or military-linked targets", Peskov said.
Russia's aerial attacks on Ukraine have repeatedly hit civilian areas.
Elsewhere, in Ukraine's central Dnipropetrovsk region a Russian guided bomb strike killed a seven-year-old girl and wounded four other people, including an 11-year-old girl, all members of the same family, regional head Oleksandr Hanzha said.
The attack used "high-precision long-range weapons" and drones on "military industry facilities and fuel and energy complexes in Kyiv and the Kyiv region, as well as military airfield infrastructure in four other regions of Ukraine", the Russian defence ministry's statement said.
It published a list of targets it said the barrage hit, mostly plants manufacturing and assembling Ukrainian drones, missiles and components.
Russia fired 74 missiles, 24 of them ballistic, and 496 drones of various types in the attack, Ukraine's air force said.
Sybiha urged countries not to delay decisions on supplying air defence systems and missiles.
He rejected any Russian attempts to justify the strikes as retaliation for Ukraine's long-range attacks, saying Ukraine was exercising its right to self-defence under Article 51 of the U.N. Charter while Russia remained the aggressor.
Ukrainian forces struck one of Russia's largest oil refineries overnight in the Nizhny Novgorod region east of Moscow, starting a fire, Ukraine's General Staff said.
Also, Ukrainian forces struck a railway bridge over the Siverskyi Donets River in the Russian-occupied Luhansk region that was used to transport Russian personnel and equipment, it said.
Ukraine's recent success with drone strikes that keep Russian troops pinned down on the front line, disrupt Russian supply lines in the rear and damage oil facilities have brought a significant change in the war, Western analysts say.
