At least 27 people were hurt in Ukraine's eastern city of Dnipropetrovsk on Friday in four blasts that President Viktor Yanukovych called a "challenge to the nation" ahead of the Euro 2012 football championship.
A total of 27 people including nine children were injured in the succession of explosions and 25 of them were hospitalised, emergencies officials told AFP.
President Viktor Yanukovych vowed a firm response to the explosions, which came weeks before the European tournament what Ukraine is co-hosting with Poland.
"We know that there are victims, we understand that this is another challenge, for the entire nation," he told journalists while on a visit to a factory in the Crimea on the Black Sea.
"We will think about how to respond adequately," he said. "I think we will figure it out. It's a pity that this has happened," Interfax quoted him as saying.
Dnipropetrovsk is the home town of Yanukovych's fierce opponent, the 2004 Orange Revolution leader Yulia Tymoshenko, who is serving a jail sentence for abuse of power that has strained relations between Ukraine and the European Union.
Tymoshenko, 51, has been on hunger strike since last Friday and has said she suffered a beating in the prison where she is serving her disputed seven-year sentence.
Dnipropetrovsk is not a host city in the country's first major international sports event, but is on the route of the trophy tour that is due there on May 21.
Officials earlier said that the first blast went off in a rubbish bin near a movie theatre in the centre of the city at 11:50 a.m. (0850 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 pm (1000 GMT), the interior ministry said without providing further details.
Amateur videos shot in the city and uploaded on News-Ua.com news website showed people tending to the wounded at the tram stop surrounded by shattered glass and sheets of metal near a tram.
Twenty-five people were hospitalised by mid-afternoon, the emergencies ministry said.
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.
No information was released as to the possible perpetrators of the blasts. A similar series of explosions in eastern Ukraine in January last year were linked by officials to an attempt to extort money.
"So far nothing is clear," a high-ranking official told AFP.
The explosions went off as security services were conducting anti-terrorist drills at the Olympic stadium in Kiev, venue for several matches in the Euro-2012 including the final on July 1.