Police detained 70 people occupying a regional administration building in eastern Ukraine overnight, but pro-Moscow protesters held out in a standoff in two other cities.
Ukrainian authorities gave few details of the "anti-terrorist" operation that cleared the building in the town of Kharkiv but said two police had been wounded. Russia has denied Ukrainian charges of involvement but warned Kiev against any use of force against Russian-speakers.
Ukrainian special forces in combat gear, helmets and balaclavas and carrying Kalashnikov rifles and machine guns stood guard early on Tuesday outside the building.
A partly destroyed sign near the main door read: "Avakov — to jail", a reference to Ukrainian Interior Minister Arsen Avakov.
Ukraine's Interior Ministry was quoted by the Interfax-Ukraine news agency as saying those detained were suspected of "illegal activity related to separatism, the organization of mass disorder, damage to human health" and breaking other laws.
Russia's Itar-Tass news agency quoted Ukrainian Deputy Prime Minister Vitaly Yarema as saying there would be no storming of Donetsk's regional authority building on Tuesday.
On Tuesday about 200 people were gathered in front of the building and a group of National Guard were stood to one side.
But the situation was calm and there was no sign of any attempt to enter by force.
Pro-Russian separatists who seized a provincial administration building in the eastern Ukrainian city of Donetsk on Monday had proclaimed the region an independent republic. The move, denounced by acting Ukrainian President Oleksandr Turchinov, echoed the run-up to Russia's annexation of Crimea.
The activists demanded that a referendum be held no later than May 11 on the secession of the Donetsk region, which borders Russia, the Interfax news agency reported. The action came after two similar incidents on Sunday night in the country, when pro-Russian demonstrators occupied government buildings in the cities of Luhansk and Kharkiv.
In the Donetsk incident, an unidentified pro-Russian activist, in footage uploaded to the Internet, asked Russian President Vladimir Putin to send peacekeeping troops to 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 after the overthrow of Moscow-leaning President Viktor Yanukovich in February.
Turchinov on Monday said the separatist action in eastern Ukraine was a “second stage of Russian special operations against Ukraine” and warned that Ukraine would use anti-terrorist measures against separatists brandishing weapons, according to Reuters. He said the Russian aim is to destabilize Ukraine, overthrow its leadership and undermine its presidential election.
The U.S. envoy to the Organization for Security and Cooperation in Europe said the group has strong evidence that thousands of Russian troops are near Ukraine and not in peacetime positions, Reuters reported.
Ukrainian Prime Minister Arseniy Yatsenyuk earlier on Monday accused Russia of being behind the unrest that broke out in the country's eastern provinces on Sunday and of seeking to sow instability as a pretext for dispatching 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 people engaged in the unrest have distinctly Russian accents.
Government buildings were also seized late Sunday night in two Russian-speaking Ukrainian cities near Donetsk — Kharkiv and Luhansk — and top government officials headed there to try to quell the unrest. But as of late Monday, the buildings in all three cities remained occupied by pro-Russian activists whom authorities said were armed.
Yatsenyuk said Russian troops remain stationed within 19 miles of the border.
In Washington, the White House warned Russia against intervening in eastern Ukraine and threatened further sanctions.
"If Russia moves into eastern Ukraine either overtly or covertly this would be a very serious escalation," said White House Spokesman Jay Carney. "We call on President Putin and his government to cease all efforts to destabilize Ukraine and we caution against further military intervention."
Carney also said evidence indicated that some pro-Russian protesters in eastern Ukraine were not local residents.
"There is strong evidence suggesting some of these demonstrators were paid and were not local residents," he said.
The Russian Foreign Ministry rejected Ukraine's allegations but reaffirmed its long-held demand that Ukraine should change its constitution to turn the country into a federation with broader powers for provinces. It added that such move should also underline Ukraine's non-aligned stance 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.
On Sunday a Russian soldier shot dead a Ukrainian naval officer in eastern Crimea, according to The Guardian. The newspaper said the shooting was confirmed by Ukraine’s defense ministry and that Russian media reported that as a group of Ukrainian soldiers in the village of Novofedorivka walked past a cohort of Russian soldiers guarding an entry to the military base where the Ukrainians previously worked, an argument broke out between the two groups.
Eastern Ukraine was a strong base of support for Yanukovich, who fled to Russia in February after months of protests. About half the region's residents are ethnic Russians, many of whom believe Ukraine's acting authorities are Ukrainian nationalists who will oppress Russians. Ukraine's interim authorities deny they are infringing the rights of the ethnic Russian population.
Since Crimea held a secession referendum and then was annexed by Russia in March, calls for similar votes in Ukraine's east have emerged.
Al Jazeera and wire services