By Olzhas Auyezov
KIEV (Reuters) - Four bombs exploded at short intervals in the Ukrainian city of Dnipropetrovsk on Friday in what authorities said amounted to an "act of terrorism", wounding 27 people, including nine children.
President Viktor Yanukovich said the 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 blast occurred at a tram stop at 11:50 a.m. (0850 GMT), wounding 13 people and shattering the windows of a nearby tram and car.
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 2 people, it said. It was unclear whether there were any injuries from the fourth explosion which also took place in the downtown area.
Police said 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, 400 km (250 miles) 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. It is also the birthplace of jailed opposition leader Yulia Tymoshenko.
Bomb attacks have been rare in the former Soviet republic which, with Poland, will co-host the European football championship in June and July.
But political tension is high because of the detention of Tymoshenko who was jailed for seven years last October for alleged abuse-of-office which she denies.
Her imprisonment has prompted sharp criticism from Western governments which see her trial as political revenge by Yanukovich, who narrowly beat her for the presidency in February 2010.
(Reporting by Pavel Polityuk and Olzhas Auyezov; Editing by Richard Balmforth)