Klitschko stated that, according to initial reports, a grenade had been detonated at a café.
“Emergency services are on their way to the scene; details to follow,” the mayor said in a Telegram post.
Later, the mayor confirmed the explosion was caused by a training grenade, causing no injuries or any significant damage to the premises.
Journalist Kristina Berdynskykh said she was at the ZigZag café when the grenade exploded.
“I had just finished writing an article and went to the ZigZag bar to meet with friends and take a break from the news,” she wrote on Facebook.
“I got distracted for an hour, and then a training grenade exploded in the bar. Quite an impression; my ears are still ringing.”
The police are yet to determine the exact nature of the incident.