"I am sure our soldiers will give a response to Russia for this terrorist attack," President Volodymyr Zelenskiy said in his nightly video address, delivered early on Sunday at the end of a visit to Sweden. "A notable response."
He said that of the 144 people injured, 15 were children, and named the girl killed as Sofia. Fifteen others were police officers, Interior Minister Ihor Klymenko said on Telegram. Klymenko said most of the victims were in vehicles, crossing the road, or returning from church.
Regional governor Viacheslav Chaus said 41 people were in hospital on Saturday.
Zelenskiy said the strike on Chernihiv, a city of leafy boulevards and centuries-old churches about 145km north of Kyiv, coincided with the Orthodox holiday of the Feast of the Transfiguration of the Lord.
Debris was scattered across a square in front of the damaged theatre and surrounding buildings, where parked vehicles were heavily damaged. A 63-year-old who only gave her first name, Valentyna, showed the damaged balcony in her apartment opposite the theatre.
"It is horrific. Horrific. There were wounded, ambulances and broken glass in here. Nightmare. Just nightmare," she said.
The roof of the neoclassical theatre was torn off by the strike.
Russia has attacked Ukrainian cities far from the frontline with missiles and drones as part of the full-scale invasion that began in February 2022.
People leaving church and others passing by were among those hurt when the missile hit the theatre, where a meeting was taking place, Chaus said.
Law enforcement agencies were looking into how Russians became aware of the event, which he said included business and community representatives but Ukrainian media reported involved drone manufacturers. Both sides have widely used drones on the battlefield.
An event organiser said all the participants, including engineers, members of the military and volunteers, were asked to go to an air raid shelter in the theatre when the alarm sounded, but some people went outside.
"All those who took shelter remained safe," Maria Berlinska, a co-founder of the Dignitas Fund, whose fundraising includes money for drones for the frontline, said in a Facebook post.
A wounded woman said her friend pulled her out of one of the damaged buildings after a part of the ceiling fell on her head.
The streets were stained with blood trails and strewn with scraps from first-aid supplies that had been used to treat the wounded.
Meanwhile, Russia's Defence Ministry said on Saturday it had destroyed a Ukrainian drone in the Belgorod region and jammed another near Moscow.
A third struck a military airfield in the Novgorod region, damaging a warplane, it also said.
