Four bombs exploded in a city in eastern Ukraine on Friday, wounding 27 people, including nine children, in what authorities described as an 'act of terrorism'.

President Viktor Yanukovich said the blasts in Dnipropetrovsk, six weeks before Ukraine plays host to the European soccer championship, represented 'a challenge ... to the whole country'.

Bomb attacks are a rare occurrence in the former Soviet republic and the blasts one after the other traumatised people in the city, one of Ukraine's main industrial centers with a population of around 1.3 million.

"I had just stepped out of my house when I saw people run down the street screaming 'Explosions!'," said 50-year-old local journalist Tetyana.

"There is panic in the city. People are taking their children out of schools and rushing to their homes," she said.

Police said the bombs had been left in rubbish bins at various points in the city. "No-one has been detained yet," a police spokesman said.

Authorities offered no immediate explanation on who may be responsible. But the prosecutor general's office formally opened a criminal case classifying the blasts as an 'act of terrorism'.

Prime Minister Mykola Azarov said the attacks could be exploited by his government's critics. "It plays into the hand of the forces that want to destabilize the situation in the country," he wrote on his Facebook page.

Dnipropetrovsk, 400 km (250 miles) southeast of the capital Kiev, is not one of the four Ukrainian cities where Euro-2012 soccer tournament matches will be played.

But the bomb blasts will unsettle Ukrainian authorities who have expressed confidence they will be able to provide full security for the tens of thousands of European soccer fans expected to converge on the country in June and July.

"I think the people who committed this brutal crime... are also accomplices to an attack on the image of our country ahead of the Euro-2012," Ukrainian football federation head Hryhory Surkis said in a statement.

Interior Minister Vitaly Zakharchenko immediately left for Dnipropetrovsk and ordered extra internal troops to the city to boost regular police. Parliament went into emergency session.

UEFA, European soccer's governing body, said it remained confident in the Ukrainian authorities' ability to stage a 'smooth and festive tournament' despite the bombings.

POLITICAL TENSION
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. The fourth explosion, close to the site of the first one, caused no casualties, said the ministry whose estimate of injuries was lower than that of police.

Very few people could be seen on the street where the first and the fourth blasts took place and which residents say was usually lively on a Friday night.

Shrapnel from one of the explosions tore off pieces of bark from nearby trees at heights where it could also hit a human.

"Nothing like this has happened before," said Volodymyr Klitchko, 23. "Dnipropetrovsk used to be a quiet city."

Outside of the city, policemen wearing bulletproof vests and armed with Kalashnikov assault rifles stopped cars for checks.

Dnipropetrovsk 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.

It 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.

Political tension is high in the country 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.

Ukraine's neighbor, Russia, offered to help the authorities investigate the bombings. Poland, another neighbor and Ukraine's co-host of Euro-2012, said the bombings were an exceptional event.

"We have to treat the issue very seriously, not only because it's a tragedy, we've heard about casualties, many people wounded," Polish Prime Minister Donald Tusk told reporters.

"Any attack in our region is something rather exceptional, and I think we can talk of an attack at this point, while the context of the Euro 2012 makes us especially sensitive to such events."