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 Monday to regain control of those buildings.
Outside the Donetsk building, a barricade of car tires and razor wire was built up to thwart police from retaking it. 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.
"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.
Ukrainian Prime Minister Arseniy Yatsenyuk also accused Russia of being behind the unrest as a pretext for sending troops across the border.
"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.
"If the political forces that call themselves the Ukrainian government continue to take irresponsible attitude to the fate of the country and its people, Ukraine will inevitably face new difficulties and crises," the ministry said in a statement.
Yatsenyuk said Russian troops remain stationed within 30 kilometers (19 miles) of the border.
Eastern Ukraine was the heartland of support for ousted President Viktor Yanukovych, who fled to Russia in February after months of protests. About half of the region's residents are ethnic Russians, many of whom believe Ukraine's acting authorities are nationalists who will oppress them. Ukraine's interim authorities deny they are infringing upon the rights of ethnic Russians and there have been no evidence of Russians facing harassment in Ukraine.
Since the Crimean Peninsula held a secession referendum and then was annexed by Russia in March, calls for similar votes in Ukraine's east have emerged. But one witness in Donetsk said the crowd Sunday included large numbers of masked men who behaved more aggressively than at previous pro-Russian rallies.
Ukraine's Security Service over the weekend said it had detained a 15-strong armed gang that was planning to seize power in the eastern province of Luhansk. The agency said it seized 300 assault rifles, an anti-tank grenade launcher, a large number of grenades, five handguns and firebombs.
It was unclear whether that group was linked to those behind Sunday's disturbances.
The international community has expressed growing anxiety over the large Russian troop movements along the Ukrainian border. NATO says up to 40,000 Russian troops have mobilized and present a distinct threat to Ukraine. Russia says it has the right to move its troops wherever it wants on its own territory.
In a video posted on the Internet, an unidentified pro-Russian activist in the Donetsk government headquarters asked Russian President Vladimir Putin to send peacekeeping troops into the region.
"Without your support, without the support of Russia, it will be hard for us to resist the Kiev junta on our own," he said, referring to the interim authorities that took power in Ukraine after the overthrow of Yanukovych.
But a senior Russian lawmaker suggested Monday that such a move was not imminent. Viktor Oserov, head of the defense committee in the Russian parliament's upper house, said Russia cannot send peacekeepers to Ukraine without a resolution by the U.N. Security Council, according to Interfax.
The ITAR-Tass state news agency cited the deputy speaker of Russia's upper house of parliament, Ilyas Umakhanov, as saying he didn't see the situation in Donetsk as a replay of Crimea.
"I don't think this situation automatically reflects what happened in Crimea ... from the judicial, historical and legal points of view, it demands a separate assessment," Umakhanov said.
Russian news agencies also reported Monday that prosecutors have opened a criminal investigation into the death of a Crimea-based Ukrainian army major who was shot and killed by a Russian soldier on Sunday.
Russian reports said Ukrainian soldiers in the village of Novofedorovskoye were on their way home when they passed Russian soldiers guarding the military base where they previously worked, and an argument kicked off between the two groups.
Jim Heintz and Vladimir Isachenkov in Moscow contributed to this report.