Dozens of inmates in a Ukrainian prison rioted, set a fire and took guards and nurses hostage in an hours-long standoff on Monday that left four guards injured, officials said.

The incident took place in Odessa, a Black Sea port city, where black smoke was seen billowing over the prison compound in the morning.

The Ukrainian ombudswoman for human rights, Lyudmila Denisova, said that several dozen inmates rioted against prison administrators and took three prison guards and three nurses hostage.

Four prison guards were injured in clashes with the inmates and it took police and investigators four hours to enter the prison and end the standoff, Ukrainian police said.

Denisova said that the riot broke out after the guards started searching the cells for banned possessions such as drugs.

Vyacheslav Abroskin, deputy chief of the Ukrainian National Police, blamed “criminal gangsters” for inciting the inmates to seize the prison administration building and set it on fire.

Five inmates suspected of instigating the riot have been removed from the facility, Abroskin said.