At least four blasts have been reported in the eastern Ukrainian city of Dnipropetrovsk.
A spokeswoman for the Emergency Situations Ministry said a total of 27 people, including nine schoolchildren, were injured in the explosions.
Officials were quoted as saying at least one of the blasts was caused by a suspected explosive device placed in a garbage bin.
A spokesman for the Prosecutor-General's Office said investigators were treating the blasts as suspected "terrorist acts."
President Viktor Yanukovych, on a visit to Crimea, called the blasts "a major challenge for the entire country."
Interior Minister Vitaliy Zakharchenko was flying to Dnipropetrovsk to investigate.
Officials said the first blast occurred at a tram stop in the center of the city, caused by a bomb in a trash bin.
At least three more blasts in crowded areas followed, including one near a railway station.
All the blasts occurred in crowded downtown areas.
With reporting by Reuters, AFP, and Interfax