Four bombs have exploded at short intervals in the Ukrainian city of Dnipropetrovsk in what authorities said amounted to an "act of terrorism", wounding 27 people, including nine children.
President Viktor Yanukovich said Friday's blasts, from bombs planted in rubbish bins, represented "a challenge ... to the whole country".
The authorities were considering how to respond "appropriately", news agencies quoted him as saying.
"The first bomb seemed to have been placed in a rubbish bin just next to a tram stop near the opera house. That blast went off at 11:50am [08:50GMT] when a tram went by.
The second explosion came 30 minutes later next to a cinema, injuring 11 people, nine of them children, the Emergencies Ministry said in a statement.
The third blast followed shortly afterwards next to a park, injuring two people, it said.
A fourth blast was also heard in the city center, the statement said. It was unclear whether anybody was injured in the fourth explosion.
Police confirmed the blasts had been caused by explosive devices planted in trash bins.
Interior Minister Vitaly Zakharchenko immediately left for Dnipropetrovsk to head an investigation, police said.
"No-one has been detained yet," a police spokesman said.
Authorities offered no immediate explanation of any motive or say who they thought could be behind the bombs. But the prosecutor general's office said it had launched a criminal case classifying the blasts as an "act of terrorism".
Dnipropetrovsk, 400km southeast of the capital Kiev, is one of the former Soviet republic's biggest industrial hubs and was a key centre of the nuclear, arms and space industries in Soviet times.
The city provided a springboard for former President Leonid Kuchma, who was in office from 1994-2005, to rise to power.
Dnipropetrovsk is also 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 European football championship that Ukraine will co-host with Poland in June and July, but is on the route of the trophy tour that is due there on May 21.
Bomb attacks have been rare in the former Soviet republics, however, 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 news agency.
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.
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