KIEV, Ukraine - Four explosions rocked an eastern Ukrainian city on Friday, injuring 27 people. Authorities say it was a terrorist attack but an opposition lawmaker claims it could be a government plot to divert attention from the imprisonment of opposition leader Yulia Tymoshenko.

Top law enforcement officials rushed to Dnipropetrovsk, 250 miles southeast of Kiev, to investigate but there was no immediate claim of responsibility. The violence undermined Ukraine's security just weeks before it co-hosts the European soccer championships in June.

Many Ukrainian officials called the blasts terrorist attacks.

President Viktor Yanukovych vowed to investigate and punish the perpetrators.

"This is yet another challenge for us, for the whole country," Yanukovych told reporters in televised comments.

However, the opposition party led by the jailed former prime minister, Yulia Tymoshenko, suggested that Yanukovych's government may have organized the blasts to deflect the world's attention from Tymoshenko's imprisonment and reported abuse.

The president's office declined to comment on the opposition charges. But Prime Minister Mykola Azarov commented on his Facebook page that the blasts "are profitable to those forces that are interested in destabilizing the situation in the country."

The first of four explosions rocked a tram stop shortly before noon, injuring 13 people, said Emergency Situations Ministry spokeswoman Yulia Yershova. The bomb was planted in a garbage bin.

The second bomb, also in a garbage bin, went off about 40 minutes later near a movie theater and a trade school, injuring two adults and nine teenagers. A third blast in the city center wounded three people, and a fourth, also downtown, caused no casualties.

Tymoshenko, 51, is serving a seven-year prison term on charges of abuse of office in a case criticized by the West as politically motivated.

She has been on a hunger strike for a week to protest the alleged prison abuse. Prison officials deny mistreating her. But photos taken by Ukraine's top human-rights official, Nina Karpachova, of Tymoshenko in bed in her jail cell show splotches on her abdomen and lower arm.