At least seven people including a six-year-old child have died and more than 140 injured in a missile strike on Chernihiv. Zelenskyy shared footage of the destruction showing debris scattered on the floor and a body slumped in a car. Meanwhile, a Ukrainian drone strike has caused a fire at a Russian military airfield.
Fifteen children were among 129 people wounded, the interior ministry said. At least 25 people were in hospital.
Among the victims were people who had been celebrating an Orthodox Christian holiday at church, the ministry said.
A main square and a university building were also damaged in the attack.
The UN called it "heinous".
Chernihiv is located around 50km from the border with Belarus. It was besieged by Russia at the beginning of the invasion.
The city's imposing theatre was hit directly. Tiles were blown off the roofs of neighbouring buildings with one catching fire 100 metres away.
The theatre was hosting a gathering of drone manufacturers, the acting mayor of Chernihiv told the BBC.
The city centre of Chernihiv is a popular area for people to stroll around, especially on the weekend, locals told the BBC.
Anna Zahreba, the manager of a Crimean Tatar restaurant just across the street from the theatre, said her staff were getting ready for a busy day when the missile hit.
Three days of mourning have been announced in the city.
Moscow is yet to comment.
Elsewhere, Russia has claimed that a Ukrainian drone hit a military airfield in the northwest Novgorod region, causing a fire that was quickly put out.
One plane was damaged but no casualties have been reported, it added.
Ukraine has not commented on the alleged drone attack.
Meanwhile, Kyiv's air force said the Ukrainian military had shot down 15 out of 17 Iranian-made Shahed drones launched by Moscow in an overnight strike.
