At least 27 people were injured Friday in the eastern Ukrainian city of Dnipropetrovsk in four successive blasts that President Viktor Yanukovych called a challenge to the nation.
"There is a total of 27" injured in three explosions in the city of about a million people east of the capital Kiev, said an emergencies ministry official, adding there was no immediate information on the fourth blast.
The explosions in Dnipropetrovsk, home town of Yanukovych's jailed opponent and 2004 Orange Revolution leader Yulia Tymoshenko, come ahead of the Euro 2012 football championship which Ukraine is co-hosting this summer.
Officials earlier said that the first blast went off in a rubbish bin near a movie theater in the center of the city at 11:50 a.m. local time (08:50 GMT).
A second blast followed 40 minutes later and the third, in another busy central street, 15 minutes after that, emergencies officials said.
The fourth explosion went off at 1:00 p.m. local time (10:00 GMT), the interior ministry said without providing further details.
"Police are looking into the casualties and circumstances" of the last two blasts, it said in a statement on its website.
Ukrainian prosecutors launched a probe into possible acts of terrorism, Interfax news agency reported. Interior Minister Vitaly Zakharchenko quickly left for Dnipropetrovsk to oversee the investigation in person.