Thousands of anti-government protesters remained in the main square in Ukraine's capital, after a day of fierce clashes with riot police left at least 26 people dead in the worst violence in three months of political demonstrations.
Ukraine's President Viktor Yanukovych has blamed opposition leaders for the violence, which escalated as riot police charged the main opposition protest camp late Tuesday in Kyiv.
Police and opposition representatives said many of those killed in the unrest were hit by gunshots. Dozens of the injured were serious condition. Nine of the dead were police officers.
The head of Ukraine's Security Service announced Wednesday it had launched an "anti-terrorist operation" to restore order in the country.
In a statement posted on the service's website, chief Oleksandr Yakimenko said municipal buildings, security offices and arms depots had been seized. He said 1,500 firearms and 100,000 rounds of ammunition wound up "in the hands of criminals" during the past day.
"Radical extremist groups and their actions are a real threat to the life of millions of Ukrainians," Yakimenko's statement read.
Protesters in the western Ukrainian city of Lviv seized various government buildings, including the local security service headquarters.
European Union foreign ministers will hold an emergency meeting Thursday in Brussels, where they are expected to agree on sanctions against those responsible for the violence in Ukraine. Speaking Wednesday at a joint press conference with her French counterpart, German Chancellor Angela Merkel said both countries agree the European Union should impose sanctions on those behind Ukraine's worst violence to date.
Earlier Wednesday, Ukrainian opposition leader Vitali Klitschko met with President Yanukovych for talks, but the meeting failed to produce a breakthrough. Klitschko said the president demanded protesters "stop the standoff" and unconditionally clear the square.
Klitschko urged the pro-Western demonstrators occupying Kyiv's Independence Square - also known as Maidan - to defend their positions. Thick black smoke could be seen rising from the barricades encircling the protest camp Wednesday.
U.N. Secretary-General Ban Ki-moon has released a statement saying he is "shocked and gravely concerned" about the escalation of violence and said use of violence by either side is unacceptable.
Ukrainian champion pole vaulter and head of Ukraine's Olympic Committee, Sergei Bubka, made a public appeal for peace Wednesday, calling on both sides to observe the "Olympic truce," a voluntary halt to violence during the international games that take place every two years.
The United States has issued a travel warning for its citizens in Ukraine, warning them to "maintain a low profile" while in the capital and be prepared to remain indoors at night, if necessary to avoid the clashes.
U.S. Vice President Joe Biden phoned Mr. Yanukovych late Tuesday to express "grave concern" about the violence, calling on the Ukrainian president to pull back security forces and exercise maximum restraint. Biden told Mr. Yanukovych his government bears "special responsibility" to resolve the crisis.
Anti-government protests have been building for weeks, with activists calling for Mr. Yanukovych's ouster after he backed away from a trade deal with the European Union in favor of closer ties with Russia.
Protesters, some armed with clubs and wearing helmets and body armor, attempted to stand their ground in central Kyiv, hurling firebombs and stones at police as plumes of smoke billowed from burning tents and piles of tires and wood.
Security forces have been steadily gaining ground in the square, where thousands of protesters remain, hearing speeches from their leaders and singing the national anthem.
Meanwhile in Russia, Kremlin spokesman Dmitry Peskov condemned the deadly Ukraine protests as a "coup attempt." He denied claims President Vladimir Putin was giving advice to Ukraine's president on how to handle the crisis and reiterated Moscow would not interfere with the Ukraine's internal affairs.