Further seizures of official buildings have been reported in east Ukraine as tensions between the new government and pro-Russian protesters rise.
Gunmen occupied a police station and a security services building in the town of Sloviansk. Official buildings in Druzhkovka were reportedly taken over.
A Donetsk police chief quit after pro-Russian crowds marched on a police station demanding his resignation.
The interim interior minister says firing has broken out in Kramatorsk.
Arsen Avakov said exchanges of fire erupted as gunmen tried to storm local administration buildings in Kramatorsk, in the Donetsk region.
The new government in Kiev accuses Moscow of orchestrating the unrest.
Interim Foreign Minister Andrei Deshchytsia urged Moscow to end "provocative" actions by its agents.
Eastern Ukraine has a large Russian-speaking population and has seen a series of protests since the ousting of pro-Russian President Viktor Yanukovych in February.
Protesters in largely Russian-speaking Donetsk, 130km (80 miles) from Sloviansk, have been occupying government buildings for days and demanding a referendum on becoming part of Russia.
A similar move prompted a Russian takeover of Ukraine's Crimea region earlier this year.
The US and EU have imposed sanctions on Russian and Crimean people they say were connected with the takeover.
Russia has denied responsibility for the protests in eastern Ukraine, but Western nations have expressed concern over a build-up of Russian troops along the border.
Trouble continued in several towns and cities on Saturday despite the Kiev government setting a deadline of Friday for all occupations to end.
Interim Prime Minister Arseniy Yatsenyuk visited Donetsk on Friday and attempted to placate anti-government groups by guaranteeing that no restrictions would be put on the use of the Russian language.
Regional police spokesman Ihor Dyomin described how armed men were bussed to the police station in Sloviansk.
"Six or seven unknown persons got out. They fired several shots in the air and attempted to storm the police department," he said.
He added that "people in camouflage uniform" and with weapons" were inside the building.
Mr Avakov promised to deal with the attackers.
"The response will be very tough because there is a difference between protesters and terrorists," he said in Ukrainian on his Facebook page.
The Interior Ministry said the aim of the attack was to seize arms from the police station, where some 40 automatic rifles and 400 pistols were stored along with ammunition.
One of those who seized the police station said the group had gone to Sloviansk to fight people who represented the illegal authorities of interim President Oleksandr Turchynov, according to a video posted on Youtube.
Interviewed in Russian, the man said they represented the People's Front of Donbass - the industrial Donets Basin region in eastern Ukraine.
He said those involved were peaceful Donbass residents and "not fascists".
The BBC's David Stern, who was briefly in Sloviansk on Saturday, said from what he saw the take-over was well-organised and the gunmen were quickly spreading their control throughout the town and beyond.
The roadblocks were being reinforced with more tyres as he left, and the people manning them were well-armed, he said.
Late on Saturday, pro-Russian activists seized the town administration in Druzhkovka, according to RBK-Ukraine media sources.
The occupation by pro-Russian groups of the local government building in the regional capital Donetsk is meanwhile continuing.
Alexander Gnezdilov, the protesters' unofficial spokesperson, told the BBC the group that seized Sloviansk police HQ was "an independent group who are supporting the Donetsk protest".
In Donetsk itself hundreds of pro-Russian protesters marched towards a police station demanding the resignation of a regional police chief, Kostyantyn Pozhydayev.
He later told the protesters he was standing down.
A Reuters witness said the Ukrainian flag above the police headquarters in Donetsk had been replaced by a separatist flag.