Russia unleashed numerous air strikes on Kyiv overnight in what officials said appeared to be the largest drone attack on the city since the start of the war.
Of the 54 drones launched, 40 targeted the capital, killing two people and wounding three.
"It's the most important drone attack against the capital since the start of the invasion" in February 2022, the regional military administration said on Telegram.
The attack "took place over several waves and the air raid alert lasted more than five hours".
"According to preliminary reports, more than 40 Russian drones were destroyed by air defence" systems over Kyiv, the administration added.
The 54 attack drones were launched "from the regions of Briansk and Krasnodar" in Russia, said the air force.
Moscow was targeting "military installations and critical infrastructure in the centre of the country and in particular the Kyiv region", it said.
Ukrainian President Volodymyr Zelenskyy hailed his country's air defence forces and rescue services on Sunday, thanking them for their response to the attack.
"You look up to destroy enemy missiles, aircraft, helicopters and drones. Every time you shoot down enemy drones and missiles, lives are saved ... you are heroes!" he said, also thanking rescuers.
The predawn attacks came on the last Sunday of May when the capital celebrates Kyiv Day, the anniversary of its official founding 1,541 years ago.
The day is typically marked by street fairs, live concerts and special museum exhibitions – plans for which have been made this year too, but on a smaller scale.
“The history of Ukraine is a long-standing irritant for the insecure Russians,” Andriy Yermak, the head of Mr Zelenskyy's office, said on his Telegram channel.
Preliminary information indicated the air raid was the largest drone attack on Kyiv since Russia's invasion, said Serhiy Popko, the head of Ukraine's military administration.
He said Russia used the Iranian-made Shahed drones in the attack.
“Today, the enemy decided to 'congratulate' the people of Kyiv on Kyiv Day with the help of their deadly UAVs [unmanned aerial vehicles],” Mr Popko said on the Telegram messaging app.
Several districts of Kyiv, by far the largest Ukrainian city with a population of around three million, suffered in the attacks, officials said, including the historic Pecherskyi neighbourhood.
The reports come as Kyiv says it is finalising plans for a major counteroffensive to recover lost territory, including the Crimea peninsula, occupied by Moscow's forces.
But the drones can also strike hundreds of kilometres inside Russia.
On Saturday, a building from where an energy pipeline is administered was damaged by two drones in western Russia, governor Mikhail Vedernikov said.
In the past week there has also been an unprecedented two-day incursion from Ukraine claimed by two anti-Kremlin groups, with Russia using its air force and artillery to push back the fighters.
Meanwhile, Russia's ambassador to the UK has warned of an escalation to the war in Ukraine as Britain and other western allies commit more weapons to aid Mr Zelenskyy.
Speaking to the BBC's Laura Kuenssberg, Andrei Kelin said the actions of Nato countries, "especially" the UK, risked lengthening and triggering a "new dimension" in the war.
He said Russia had "enormous resources" and despite the war already raging for more than a year, he claimed his country had not yet started to "act very seriously".
Mr Kelin said: "Russia is 16 times bigger than Ukraine. We have enormous resources.
"Sooner or later, of course, this escalation may get a new dimension, which we do not need and we do not want. We can make peace tomorrow."
When asked about widespread mounting evidence of Russia's war crimes, the ambassador said Ukraine was the one committing war crimes.
He then changed tack and added the UN had no power to investigate war crimes.