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 in what Kiev called a Russian-led plan to dismember the country.
Kiev says the seizure of public buildings in eastern Ukraine's mainly Russian-speaking industrial heartland on Sunday night is a replay of events in Crimea, the Black Sea peninsula Moscow annexed last month.
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 by a grenade that was thrown. 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 kalashnikovs and machine guns stood guard early on Tuesday outside the building whose outside windows were broken.
A partly destroyed sign near the main door read: "Avakov -to jail", a reference to Ukrainian Interior Minister Arsen Avakov.
The pro-Russian protesters who also took over official buildings in Luhansk and Donetsk demanded that referendums be held on whether to join Russia like the one that preceded Moscow's annexation of Crimea.
"An anti-Ukrainian plan is being put into operation ... under which foreign troops will cross the border and seize the territory of the country," Prime Minister Arseny Yatseniuk said in public remarks to his cabinet. "We will not allow this."
Interior Minister Avakov said on Tuesday about 70 "separatists" had been detained in Kharkiv.
"An anti-terrorist operation has been launched. The city centre is blocked along with metro stations. Do not worry. Once we finish, we will open them again," he wrote on his Facebook page.
Ukraine's Interior Ministry was quoted by Interfax-Ukraine news agency as saying those detained were suspected of "illegal activity related to separatism, the organisation 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.
He said the decision was made after talks in Donetsk with the protesters involving influential and wealthy businessman Rinat Akhmetov, who is from the city.
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 siutation was calm and there was no sign of any attempt to enter by force.
Acting President Oleksander Turchinov, in a televised address to the nation, said Moscow was attempting to repeat "the Crimea scenario". He added that "anti-terrorist measures" would be deployed against those who had taken up arms.
U.S. Secretary of State John Kerry told Russian Foreign Minister Sergei Lavrov in a phone call that Washington was watching events in eastern Ukraine with great concern and any further moves by Moscow to destabilise Ukraine would "incur further costs for Russia".
Kerry "called on Russia to publicly disavow the activities of separatists, saboteurs and provocateurs" in Ukraine, the State Department said. The two discussed convening direct talks in the next 10 days between Ukraine, Russia, the United States and the European Union to defuse tensions.
The White House warned Russian President Vladimir Putin against moving "overtly or covertly" into eastern Ukraine and said there was strong evidence that pro-Russian demonstrators in the region were being paid.
Lavrov told Kerry that constitutional reform was required to resolve the crisis. Russia says this would give Ukraine's regions more powers, as it believes the rights of ethnic Russians in Ukraine are being violated.
In an article on the website of Britain's Guardian newspaper, Lavrov denied Russia was destabilising Ukraine and accused the West of the "groundless whipping-up of tension".
Separately, he warned authorities in Kiev against any use of force against pro-Russian demonstrators.
The Russian Foreign Ministry later called on Ukraine to stop massing military forces it said were tasked with suppressing anti-government protests.
"We call for an immediate halt to military preparations which could lead to an outbreak of civil war," the ministry said in a statement.
Police said they cleared the protesters from the building in Kharkiv, but in Luhansk the demonstrators had seized weapons.
In Donetsk, home base of deposed Moscow-backed President Viktor Yanukovich, about 120 pro-Russia activists calling themselves the "Republican People's Soviet of Donetsk" seized the chamber of the regional assembly.
An unidentified bearded man read out "the act of the proclamation of an independent state, Donetsk People's Republic" in front of a white, blue and red Russian flag.
"In the event of aggressive action from the illegitimate Kiev authorities, we will appeal to the Russian Federation to bring in a peacekeeping contingent," ran the proclamation.
The activists later read out the text by loud hailer to a cheering crowd of about 1,000 outside the building.
Putin announced on March 1, a week after Yanukovich was overthrown, that Moscow had the right to take military action in Ukraine to protect Russian speakers, creating the biggest confrontation between Moscow and the West since the Cold War.
The United States and EU imposed mild financial sanctions on some Russian officials over the seizure of Crimea and have threatened much tougher measures if Russian troops, now massed on the frontier, enter other parts of Ukraine.
Western European governments have hesitated to alienate Russia further, fearing for supplies of Russian natural gas, much of which reaches EU buyers via pipelines across Ukraine. Ukraine's own dependence on Russian gas gives Moscow strong leverage, especially over Ukraine's eastern industrial areas.
Russia's gas monopoly Gazprom said it had received no payments from Ukraine for money owed for gas and Kiev's arrears have now reached $2.2 billion. In previous years, gas disputes between Moscow and Kiev have hurt supplies to Europe.
NATO has halted cooperation with Russia. The Western military alliance announced on Monday it would now restrict access to its headquarters by Russian diplomats apart from Moscow's ambassador, his deputy and two support staff.
Mainly Russian-speaking eastern Ukraine, densely populated and producing much of the country's industrial output, has seen a sharp rise in tension since Yanukovich fled the country, and Kiev has long said it believes Moscow is behind the unrest.
Pro-Russian protesters briefly held public buildings in the east early last month and three people were killed in clashes in mid-March. But trouble had subsided until Sunday.
Unlike in Crimea, where ethnic Russians form a majority, most people in the east and south are ethnically Ukrainian, although they speak Russian as a first language. Eastern oligarchs who once backed Yanukovich have thrown their weight behind the government in Kiev, and the unrest there is a test of their ability to assert control.
(Additional reporting by Elizabeth Piper, Natalia Zinets and Pavel Polityuk, Writing by Richard Balmforth, Peter Graff and Timothy Heritage, editing by Elizabeth Piper and Andrew Heavens)