KIEV, Ukraine — A deadly confrontation in the Ukrainian capital eased, at least temporarily, late Wednesday when the government announced that it had reached a truce with opposition protesters who continue to occupy a square in central Kiev.

The truce announcement came after a tense day in which the government warned that it was launching an "anti-terrorist operation" and President Viktor Yanukovich replaced the armed forces chief, who reportedly had been reluctant to use the army against civilian protesters.

Yanukovich and three opposition leaders agreed on a truce after a round of urgent talks in the presidential administration building in downtown Kiev, opposition leader Arseny Yatsenyuk told reporters.

"The positive news is that the planned storming and the sweeping [of the Independence Square opposition camp] have been canceled," Yatsenyuk said after emerging from the meeting. "A truce has been called that sets the beginning for negotiations to stabilize the situation."

The news, however, was not met with much enthusiasm in the square, where the sounds of stun grenades used by police continued from time to time even after the truce was called. Riot police units were not being withdrawn from the area near the square.

At least one opposition leader said his group would not observe the truce.

Earlier, busloads of police and Interior Ministry troops poured into central Kiev, where thousands of protesters remained in the smoky expanse of Independence Square. World leaders, including President Obama, urgently called on both sides to step back from the brink — words that may ultimately have been heeded.

Violence Tuesday and Wednesday claimed more than 25 lives and left hundreds injured in the worst clashes since protests began in November. Western-leaning opposition forces are furious with Yanukovich for spurning an economic pact with the European Union in favor of closer ties with neighboring Russia, which shares a common culture with Ukraine but also has a long history of dominating it.

The West has pleaded with Ukraine to find a path to compromise, but Yanukovich appeared to take another tack Wednesday, when his security chief issued a strongly worded warning on his agency's website.

"What is happening today is a conscious use of violence by way of arson, murder, hostage-taking and intimidation ... for the sake of pursuing criminal goals," Alexander Yakimenko said in a statement published on the Ukraine Security Service website. "All of that with the use of firearms. These are not just signs of terrorism but concrete terrorist acts.

"By their actions, radical and extremist groups bear a real threat to lives of millions of Ukrainians," his statement said.

Hours later, Ukraine's UNIAN news agency said Yanukovich had fired the armed forces chief, Vladimir Zamana, and replaced him with another commander, Yuri Ilyin.

"The sudden switch can be explained by Yanukovich's desire to use the army in combating the growing protests," Vadim Karasyov, head of the Institute of Global Strategies, a Kiev-based think tank, said in an interview. "Zamana has recently hesitated … to get involved in helping to defuse the political crisis."

As dusk fell over Independence Square, several thousand protesters armed with sticks, stones and Molotov cocktails faced hundreds of police officers equipped with tear gas and stun grenades, water cannons and shotguns loaded with rubber bullets.

Firefighters continued efforts to extinguish a blaze consuming the Trade Unions building, which had served as the opposition headquarters until it caught fire during clashes Tuesday night.

Police and the opposition blamed each other for the fire.

The Ukraine Security Service statement about terrorists came on the heels of Yanukovich's speech earlier in the day in which he laid responsibility for the violence and casualties on the opposition, and claimed there had been a coup attempt.

At least one political analyst called the security agency declaration disturbing and potentially dangerous.

"The declaration of an anti-terrorist operation differs from the imposition of a state of emergency only inasmuch as it doesn't need to be declared by the president and doesn't need to be approved by parliament," Kost Bondarenko, director of the Ukrainian Policy Institute, a Kiev-based think tank, said in an interview.

Radical opposition activists, especially in western regions of Ukraine, have alarmed the government in recent days with tactics such as attacking military units and capturing a weapons storehouse in the Ivano-Frankivsk region, and taking the family of a governor hostage in the town of Lutsk.