Buses and tyres – barricades used by pro-Russian separatists – smoulder on the streets of Kramatorsk after Ukrainian troops moved in to recapture the eastern town.
The Ukrainian Interior Ministry says the security service headquarters in Kramatorsk have been taken back and are now under the control of the National Guard.
There have been reports from Kramatorsk residents of a large number of deaths and injuries on both sides.
South of Kramatorsk, Kyiv launched a pre-dawn operation to retake the rebel stronghold of Slovyansk.
The operation saw two military helicopters being downed and the deaths of at least four servicemen.
The separatists say three of their number have been killed, along with two civilians.
The unrest in Ukraine has been spreading. Hundreds of kilometres from Slovyansk, the port city of Odessa erupted into violence on Friday.
People gathered on Saturday to pay their respects after dozens of people were killed in a building fire that broke out during clashes between pro-Ukrainian and pro-Russian demonstrators.