An auditorium inside the Trade Unions Building, which has served as the de facto headquarters for the anti-government protest movement, fills with smoke and flames as the building burns. Brendan Hoffman/Getty Images

Protesters force their way into the main office of the ruling Ukrainian Party of Regions. Oleg Petrasyuk/EPA

Ukrainian riot police in front of the fire ring around Independence Square in Kiev on Wednesday, a day when more than two dozen were killed. Sergey Dolzhenko/EPA

An anti-government protester waves the national flag from the top of a statue during the fighting. Reuters

An injured man struggles to breathe as he is carried on a stretcher by anti-government protesters. Yannis Behrakis/Reuters

Activists mourn protesters killed in clashes with police in Kiev. More than 50 people have been killed over the last two days in the protest. Laszlo Beliczay/EPA

A priest in the Hotel Ukraine on Thursday near bodies of anti-government protesters killed during the clashes with riot police. David Mdzinarishvili/Reuters

Police leave their position around the Ukrainian parliament in Kiev on Friday after the country’s deputy army chief resigned in protest over government attempts to involve the army to put down the unrest rocking Ukraine. Genya Savilov/AFP/Getty Images

Ukrainian President Viktor Yanukovich accused pro-European opposition leaders on Wednesday of trying to seize power by force after at least 25 people died in the worst violence since the former Soviet republic gained independence.

European Union leaders said they were urgently preparing targeted sanctions against those responsible for a crackdown on protesters who have been occupying central Kiev for almost three months, since Yanukovich spurned a far-reaching trade deal with the EU and accepted a $15 billion Russian bailout.

On Tuesday, defiant protesters — 20,000 strong — shouted “Glory to Ukraine” after opposition leader Vitali Klitschko urged them to defend the camp on Independence Square. Burning tents lit up the night sky after thousands of riot police moved against the sprawling protest camp.

The violence was the deadliest to rock the capital in nearly three months of anti-government protests that have paralyzed Ukraine's capital in a struggle over the nation's identity. The protests in the spawling nation of 46 million have become a source of geopolitical tension, as the United States and its Western allies want Yanukovich to turn back to Europe and the prospect of an International Monetary Fund–supported recovery, while Russia accuses the U.S. and EU of overstepping.

The rioting on Tuesday began after opposition leaders accused pro-government factions in parliament of dragging their feet on a constitutional reform that would limit presidential powers — a key opposition demand.

“This is not an armed insurrection of the opposition," Sviatoslav Yurash, a spokesman for the opposition’s Euromaidan Movement, told Al Jazeera. "This is just people trying to defend themselves against the indiscriminate actions of the government.”

U.S. Vice President Joe Biden reached out to the president of Ukraine to express the United States’ concern over the growing violence. Biden “made clear that the United States condemns violence by any side, but that the government bears special responsibility to de-escalate the situation,” said a White House statement.

On Wednesday, Russian President Vladimir Putin's spokesman insisted the Kremlin was sticking to a policy of not intervening in Ukraine, although his point man has called for action to crush the protests. The Kremlin said Putin and Yanukovich spoke by telephone overnight, calling the events an attempted coup.

Moscow announced the resumption of stalled aid to Kiev on Monday with a $2 million cash injection hours before the crackdown began.

After a night of gasoline bombs and gunfire on Independence Square, black smoke billowed from a burned-out trade union building on Wednesday that protest organizers had used as a headquarters. Security forces occupied about a third of the square — the part that lies closes to government offices and parliament — with protesters pouring in to reinforce their defenses on the remainder of a plaza they have dubbed "Euro-Maidan" (Euro Square).

In a statement posted online in the early hours, Yanukovich said he had refrained from using force since unrest began but was being pressed by "advisers" to take a harder line. "Without any mandate from the people, illegally and in breach of the constitution of Ukraine, these politicians — if I may use that term — have resorted to pogroms, arson and murder to try to seize power," the president said.

He declared Thursday as a day of mourning for the dead.

Klitschko, the senior opposition leader, walked out of a meeting with Yanukovich during the night, saying he could not negotiate while blood was being spilled.

The Health Ministry, updating the casualty toll, said 25 people had been killed in the fighting in the capital, of whom nine were police officers. The police later said a 10th officer had died of his wounds. The Interior Ministry said five of the policemen had died of identical wounds from sniper fire to the head and neck. Journalists saw some hard-line protesters manning barricades armed with rifles, including one with a telescopic sight.

EU foreign ministers called an emergency meeting on Ukraine for Thursday, and European Commission President Jose Manuel Barroso said the 28-nation bloc was set to impose sanctions against those blamed for the bloodshed.

"We have ... made it clear that the EU will respond to any deterioration on the ground," Barroso said in a statement. "We therefore expect that targeted measures against those responsible for violence and use of excessive force can be agreed by our member states as a matter of urgency."

EU ambassadors were meeting in Brussels to consider steps, from travel bans to asset freezes. Neighboring Poland and former Soviet republic Latvia called for emergency EU action against the Ukrainian authorities.

The leaders of Germany and France, the EU's main powers, were meeting in Paris and were expected to issue a joint statement on the crisis later in the day. French President Francois Hollande backed Poland's call for "quick and targeted sanctions against those responsible." German Chancellor Angela Merkel was "deeply saddened" by the escalation, but a spokeswoman said "we are not yet there" when asked whether the time for EU sanctions had arrived.

Meanwhile, on Wednesday, protesters, many of them masked and in battle fatigues, were pouring onto the square from another direction and preparing to take on police for a second straight day. As priests intoned prayers from a stage on the part of the square still held by protesters, young men in hard hats were constructing forearm and knee pads — protection against baton blows. Others were pouring flammable liquid into bottles, apparently to be used as gasoline bombs.

"They can come in their thousands, but we will not give in," said a 44-year-old from the western region of Ternopil who gave only his first name, Volodymyr. "We simply don't have anywhere to go. We will stay until victory and will hold the Maidan until the end."

"We will stay until victory. We want our children to grow up in a normal country where there are civilized laws, not the laws of a prison colony," said Vitaly, 36.

Al Jazeera and wire services