Ukraine's acting president said the country will launch a "large-scale anti-terrorist operation" to resist Russia's aggression.
Oleksandr Turchynov said in a live televised address from the parliament that authorities in Kiev will not allow the "repetition of the Crimean scenario."
Russia annexed the Crimean peninsula last month after local residents voted overwhelming to secede from Ukraine and seek to join Russia in a hastily called referendum.
Turchynov reportedly called an emergency meeting of Kiev's national security council on Saturday after pro-Russian protesters seized control of government buildings in the country's eastern region.
"A meeting will take place of the Ukrainian council of security and defense because of the situation in the east of Ukraine," a spokeswoman for Acting President Oleksander Turchinov said, according to Reuters.
On Saturday, men in the uniforms of Ukraine's now-defunct riot police occupied police headquarters in Donetsk, the eastern city that is one of the flashpoints of a wave of pro-Russia protests, hours after armed men seized local police headquarters and local branch of the Security Service in a nearby city.
The unrest in Donetsk and Slovyansk, about 55 miles to the north, were the latest shows of spiraling anger in eastern Ukraine, which has a large Russian-speaking population and was also the support base for Viktor Yanukovych, the Ukrainian president who was ousted in February after months of protests in the capital, Kiev. Ethnic Russians in Ukraine's east widely fear that the authorities who took over after Yanukovych's fall will suppress them.
In Slovyansk, the mayor said the men who seized the police station were demanding a referendum on autonomy and possible annexation by Russia. Protesters in other eastern cities have made similar demands after a referendum in Crimea last month in which voters opted to split off from Ukraine, leading to annexation by Russia.
"Terrorists with weapons with be treated with zero tolerance."
- Ukraine's Interior Minister Arsen Avakov
A regional news website said another police station had been seized in the town of Krasnyi Lyman, a town near Slovyansk, but that report could not immediately be confirmed. In Kramatorsk, another town near Slovyansk, protesters flew the flag of the Independent Donetsk Republic above the local administration building, according to Kramatorsk's news website.
Witnesses said the men who entered the police building in Donetsk were wearing the uniforms of the Berkut, the feared riot police squad that was disbanded in February after Yanukovych's ouster. Berkut officers' violent dispersal of a demonstration in Kiev in November set off vast protests in the capital that culminated in bloodshed in February when more than 100 people died in sniper fire; the acting government says the snipers were police.
It was not immediately clear if the men who occupied the Donetsk police building had made any demands, but the Donetsk police chief said on national television that he was forced to offer his resignation. Interfax Ukraine reported that pro-Russian protesters had invited the former police chief to resume his duties.
"In accordance with your demands I am stepping down," police chief Kostyantyn Pozhydayev reportedly told protesters.
In Slovyansk, about 20 men in balaclavas armed with automatic rifles and pistols were guarding the entrance to the police station in the city of about 120,000 people, and another 20 were believed to be inside. They wore St. George's ribbons, which have become a symbol of pro-Russian protesters in eastern Ukraine. The ribbons were originally associated with the Soviet Union's victory in World War II.
"We are very concerned by the concerted campaign we see underway in eastern Ukraine today by pro-Russian separatists, apparently with support from Russia, who are inciting violence and sabotage and seeking to undermine and destabilize the Ukrainian state," National Security Council spokesperson Laura Lucas Magnuson said in a statement Saturday.
"We call on President Putin and his government to cease all efforts to destabilize Ukraine, and we caution against further military intervention," Magnuson said.
Eastern Ukraine, which has a large Russian-speaking population, has seen waves of protests since Kremlin-friendly Yanukovych was ousted. The protesters allege that the authorities who took over are nationalists and "fascists" who aim to suppress the ethnic Russians in Ukraine.
The predominantly ethnic Russian region of Crimea voted in a referendum last month to split off from Ukraine and was subsequently annexed by Russia in moves that West has denounced as illegitimate.
A masked guard in Slovyansk, who gave his name only as Sergei, told The Associated Press they have "only one demand: a referendum and joining Russia."
The man said they seized the building because they wanted to protect it from radical nationalists from western Ukraine and "the junta who seized power in Kiev."
"We don't want to be slaves of America and the West," he said. "We want to live with Russia."
The Interior Ministry said in a statement that the attackers' goal was to seize arms from the police station. They said there were about 40 automatic rifles and 400 pistols as well as ammunition inside.
A video from the scene saw one man carrying a sniper rifle. An AP reporter saw another man loading the magazine of a pistol at the police station.
Interior Minister Arsen Avakov pledged a "very tough response" to the seizure while local media reported special forces dispatched to the area.
"The response here will be very tough because there is a huge difference between the protesters and terrorists," Avakov said.
"Terrorists with weapons with be treated with zero tolerance," he said, according to The Wall Street Journal.
Local sympathizers brought tires to the police station to start building barricades.
Gunshots rang out in the background in a video from the scene after an armed man shouted to a cameraman to stop recording. No casualties were immediately reported.
An Associated Press crew saw about 10 men wearing camouflage and carrying automatic rifles at the entrance to the town late Saturday afternoon. The men were building a barricade with tires.
Kiev has offered amnesty to protesters if they surrender their weapons and leave the buildings, The Wall Street Journal reported.
The Kiev authorities and the United States have accused Russia of fomenting the unrest in the east and seeking to use it as a pretext for sending in troops. Russia has massed forces in areas near the Ukrainian border.
But Mayor Nelya Shtepa told the AP that she held talks with the protesters and said they were local residents, not Russians.
"They told me: 'We don't have anything against you,'" she said, adding that the men said they "want to be heard, want a dialogue with authorities in Kiev."
Protesters, who have held the administration building in Donetsk since Sunday, initially called for a referendum on secession but later reduced the demand to a vote on autonomy within Ukraine with the possibility of holding another later on whether to join Russia.
Russia's Foreign Ministry on Friday warned the Ukrainian government against using force against protesters, saying that such action would derail the talks on settling the crisis between the United States, the European Union, Russia and Ukraine set for next week, as well as any other diplomatic efforts. It lashed out at the U.S. warning to slap more sanctions on Russia in case of an escalation of the conflict, saying that "an escalation is only and exclusively possible if Kiev dares to do so, relying on massive support of the U.S. and the EU."
The Associated Press and Reuters contributed to this report.