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The protests were prompted by suspicions one of the policemen involved was being protected because of family connections.
Several hundred people took to the streets in Vradiyevka, 400 km south of the capital Kiev, after reports circulated of the attack on a 29-year-old shop assistant who said she was beaten and raped by two policemen after being grabbed on a street and forced into a taxi.
Video footage at the scene showed police cowering back as mobs tried to force their way into the main police building and used metal bars to beat in windows and a metal perimeter fence. Police used what appeared to be pepper-spray to force them back.
Part of the building was ablaze in the early hours of Tuesday after petrol bombs were thrown. Angry crowds were still demonstrating in the town in mid-afternoon despite a visit by the regional governor to calm them.
"What happened last night is a wake-up call," Vitaly Klitschko, the world boxing champion-turned-politician, told parliament which interrupted a discussion on European integration to hear reports on the overnight unrest.
"The impunity of law enforcement authorities has caused a people's rebellion," Klitschko said.
President Viktor Yanukovich, who is already at odds with the EU over the jailing of his main political opponent former prime minister Yulia Tymoshenko, told security chiefs urgently to investigate the incidents and violence.
Authorities announced that the regional prosecutor as well as the heads of the regional and town police had been sacked.
Her face swollen and disfigured by bruising, Iryna Krashkova told a television channel from her hospital bed that she had been grabbed late at night on June 26 as she walked home from a discotheque.
She was driven into woods and raped by two policemen, both of whom she knew by sight, while a taxi driver stood by and did nothing. "They wrenched off my ear rings and threw me out of the car," she said.
Interior Minister Vitaly Zakharchenko pledged to carry out a "fast, full and unbiased investigation".
He confirmed that one policeman and the taxi driver had been detained. But he said a second policeman whom Krashkova had identified had not been arrested because records showed he had been on duty at the time which suggested he could not have taken part in the crime.
He said however there would be an expert investigation into all the circumstances.
He acknowledged that local police had tried to minimise the affair by initially recording the attack as a minor one involving only slight injuries to the victim. His office had reacted slowly because of "poor information" from regional police.
In a similar gang-rape in March 2012 in the Ukrainian provinces, Oksana Makar, 18, died after being raped, half-strangled and then set on fire.
That crime also sparked street protests when police initially released two of her three suspected attackers because their parents had good political connections. The three men were subsequently re-arrested, convicted and are now serving jail sentences.