Anti-government protesters on Saturday seized a regional administration building, news reports said, and officials warned that police could storm the Kiev city hall to free two policemen allegedly captured by demonstrators.
Protesters have occupied the city hall for nearly two months and turned it into a makeshift dormitory and headquarters. Protesters deny they are holding the officers.
A ministry statement warned that police would storm the building if the two officers were not released. It said another officer who had been injured while being seized had been released and was hospitalized in serious condition.
The city hall is only a few hundred meters yards from both the site of protracted clashes between police and protesters over the past week and Independence Square, where demonstrators have set up an extensive tent camp and conducted round-the-clock protests since early December.
An attempt by police to storm the building would likely set off new clashes.
Protesters on Saturday morning seized the headquarters of the energy ministry, but left it several hours later. Energy Minister Eduard Stavitskiy was quoted by the Unian news agency as saying that all the country's nuclear power facilities were put on high alert after the seizure.
In Vinnitsya, about 110 miles southwest of Kiev, hundreds of demonstrators stormed the local administration building, Ukrainian news agencies said. Until the past week, the protests had been centered in Kiev with only smaller demonstrations elsewhere, but since the Kiev clashes began on Sunday, a score of local government buildings have been seized in the country's west, where support for President Viktor Yanukovych is thin.
Yanukovych has refused protesters' demand to resign and call early elections, offering only minor concessions to the opposition Friday. Violent clashes then resumed in Kiev's government district, with protesters pelting rocks and fire bombs at police, who responded with stun grenades, tear gas and rubber bullets.
On Saturday morning, the clash site was tense, with demonstrators milling about, many of them bearing clubs, metal rods and large sticks. They watched as black smoke billowed from a barricade of burning tires, but there was no violence.
Interior Minister Vitali Zakharchenko, who is in charge of the police and is one of the figures most despised by the protesters, said other countries "must not close their eyes" to rising extremism in Ukraine. The country has come under wide criticism from the West during the protests, particularly after at least two demonstrators died of gunshot wounds in the clashes this week.
"The events of recent days in the Ukrainian capital showed that our attempts to peacefully resolve the conflict without resorting to forceful opposition remain futile," Zakharchenko said.
Yanukovych has called a special parliament session for Tuesday.
He said on Friday that the session would consider a government reshuffle, amnesty for many of the arrested protesters and changing recently passed harsh new laws cracking down on protests.
The new laws were a critical factor in prompting the last week's violence, in contrast to the determined peacefulness of most of the previous weeks of protests.
The holding company of Rinat Akhmetov, a powerful tycoon whose support has been important to Yanukovych, on Saturday issued a statement calling for a peaceful resolution of the crisis, saying "The most important thing is that the route of force will not find an exit."
AP writer Yuras Karmanau in Minsk contributed to this story.