Later in the day, pro-Russia activists took control of the state TV center in this regional capital without firing a shot. Members of a separatist movement called the Donetsk People's Republic, aided by a fight club from the eastern city of Kharkiv, stormed the broadcast facility, saying they were sick of watching news aired through the prism of their enemies in Kiev and demanding an undiluted stream of Russian programing.
The day's events showed eastern Ukraine slipping further into chaos, with armed separatists openly defying state authority and local police either folding in sympathy or admitting that they felt too intimidated to stop the pro-Russia groups.
There was no sign of the new government in Kiev, the Ukrainian capital, pressing ahead with its "anti-terror" police and military operation to retake buildings and checkpoints in the east occupied by pro-Russia militants.
As Moscow and Washington traded blame for a failure to halt the escalation in tension, diplomats moved to try to free the hapless European military monitors.
Human rights observers say that as many as 24 people — journalists, activists, police officers, politicians and monitors — are being held captive in makeshift jails in Slovyansk, a city in the Donetsk region, in the heart of territory controlled by the separatists.
There were also reports of vehicles being carjacked by pro-Russia activists and of a list circulating at anti-Kiev checkpoints with the photographs of 18 journalists to be arrested on the spot.
The Security Service of Ukraine said that three of its officers were captured by pro-Russia militants in the city of Horlivka, where the agents were investigating the recent torture and killing of a local politician and a university student. Both men were supporters of a unified Ukraine. Their bodies were found dumped in the river near Slovyansk.
Sunday's incidents occurred as Western nations prepared to impose fresh sanctions on Russia. Tony Blinken, the White House deputy national security adviser, said on CBS's "Face the Nation" that the new measures would be aimed at members of Russian President Vladimir Putin's inner circle, as well as "companies that they and other inner-circle people control."
The European Union is also expected to announce new measures Monday, though Europe has generally been more hesitant than the United States to punish Russia.
In Slovyansk, hard-core separatists staged a news conference to display their captives, who were in Ukraine as part of a military observer mission operating under the auspices of the Organization for Security and Cooperation in Europe.
"We wish from the bottom of our hearts to go back to our homes as soon and as quickly as possible," one of the observers, Axel Schneider, a German army colonel, said at the news conference.
The captives said that they were not "prisoners" but "guests" of the de facto mayor but that they did not know when they might be released. Their captors said they might use the hostages for a prisoner exchange.
Schneider said the men had "not been touched" and were in good health. "We are not fighters. We are diplomats in uniform. We came without weapons," he said.
The separatists, saying they had found the observers carrying maps indicating the location of checkpoints, labeled the captives "NATO spies."
German Foreign Minister Frank-Walter Steinmeier said the parading of the military officers before the news media was "repulsive" and "a breach of all the rules." He appealed to Russia to pressure the separatists to free the monitors, who were arrested alongside five Ukrainian military officers and a driver.
Late Sunday, one of the monitors was released into the custody of international negotiators, according to Michael Bociurkiw, an OSCE spokesman. The freed observer was from Sweden and was probably released because he has a serious preexisting medical condition, the spokesman said.
Support for the separatists in southeastern Ukraine is thin. The government buildings they occupy in towns and cities across the region are often defended by no more than a few dozen protesters, and they have had trouble drawing large crowds.
Polls suggest that most people in the region, while favoring greater autonomy, do not want to be absorbed by Russia.
Only a few hundred demonstrators turned out for a rally in Donetsk's Lenin Square on Sunday, despite nearly ideal spring weather and the promise of an appearance by the mysterious "people's governor."
The crowd — waving Russian and Donetsk People's Republic flags — offered tepid applause as speaker after speaker railed against the "fascist government in Kiev."
"I want to strangle a fascist with my bare hands," shouted a woman who took the stage beneath a massive black stone statue of Lenin.
Another speaker chided the leaders of the Donetsk People's Republic for not being more aggressive in attacking the Ukrainian authorities and achieving autonomy. The power to his microphone was cut off mid-sentence, and men wearing camouflage uniforms and balaclavas escorted him from the stage.
The self-appointed "people's governor," Denis Pushilin, told the crowd that his organization planned to stage a referendum May 11 on a single question: "Do you recognize the creation of the Donetsk People's Republic?"
If the answer is "yes," Pushilin said, residents will decide whether to remain autonomous or join a neighboring state — which could only be Russia, which annexed the Ukrainian territory of Crimea last month.
"I learned a lot from my Russian friends on how to be a patriot," Pushilin said.
Boris Litvinov, who is organizing the referendum, said the ballots had gone to the printers. But he conceded that the election faced logistic, political and legal hurdles. Only two cities are working closely with the group — "and half of Kramatorsk," another city, he said.
The interim governor of Donetsk, Serhiy Taruta, a billionaire steel magnate appointed to the job by the Kiev government, said it would be impossible for the pro-Russia activists to pull off a fair referendum.
"We have 2.7 million voters in the region and 5,000 polling districts, for heaven's sake," Taruta said in an interview. "I can call myself the pope. Who cares? That doesn't make it real or transparent."
As separatists stormed the TV station, pro-unity Ukrainians gathered at a Donetsk cultural center to hear from dissident Russian businessman Mikhail Khodorkovsky.
The former oil tycoon was freed from a Russian jail in December after 10 years in prison on charges that were widely seen as politically motivated.
He has since been outspoken in his criticism of Putin, and on Sunday he made common cause with Ukrainians who oppose any interference from Moscow. Ukrainians, he said, should choose to stay in Ukraine because here "you live in a democracy, more or less. But Russia is an authoritarian regime that is transforming into a totalitarian regime."
Khodorkovsky urged the Ukrainian audience — which included prominent Donetsk civic leaders — to demonstrate for Putin the cost of an invasion.
"If Putin and Russian decision-makers understand that this region is not like Crimea, and that people here will resist an invasion, they won't invade," he said to warm applause. "It's up to you to send a strong signal that you'll fight for your land and that you won't give up. Once you make that decision, you should be prepared to defend it with your blood." - By William Booth and Griff Witte (Alex Ryabchyn contributed to this report.)
