Ukrainian authorities called the move an attempt by Russia to sow unrest.
The Interfax news agency said the activists demanded that a referendum be held no later than May 11 on the possible secession of the Donetsk region, which borders Russia.
Pro-Russian crowds on Sunday had stormed government buildings in Donetsk, Luhansk and Kharkiv, all cities in Ukraine's heavily Russian-speaking east. Authorities say all the storming parties appeared to have been armed. The government was struggling today to regain control of those buildings.
Interfax cited police in Donetsk as saying one armed group fired into the air and attempted to seize the regional state television broadcaster Monday but retreated after police and guards in the building also fired warning shots into the air.
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"After that, the attackers fled to an unknown location," police were cited as saying.
Speaking in a televised address, acting President Oleksandr Turchinov called the events gripping eastern Ukraine an operation undertaken by Russia to sow instability.
"Anti-terrorism measures will be adopted against those who took up weapons," Turchinov said in Kiev, adding that parliament would convene Tuesday to consider tougher penalties for separatist actions and even ban parties that engage in separatism.
"The plan is to destabilize the situation, the plan is for foreign troops to cross the border and seize the country's territory, which we will not allow," he said, adding that those taking part in the unrest had distinct Russian accents.
The Russian Foreign Ministry rejected the allegations but reaffirmed its long-held demand that the Ukrainian government turn the country into a federation with broader powers for provinces. It added that such reforms should also seal Ukraine's non-aligned status and ensure a special status for the Russian language.