The demonstrators forced their way into into the post office in Independences Square, also known as the Maidan, after a nearby building they had previously occupied was burned down in the previous day's clashes. Against the official onslaught, thousands of activists armed with fire bombs and rocks defended the square which has been a bastion and symbol for the demonstrators.
During the night, the square was encircled by a wall of fire from burning tires. Smoke was still rising from the rose above the center of the Ukrainian capital on Wednesday afternoon.
Ukraine's top security agency on Wednesday accused protesters of seizing hundreds of firearms from its offices and announced a nation-wide anti-terrorist operation after 25 people were killed and hundreds injured hundreds in street clashes in the most unrest in the country's modern history.
The violence Tuesday was the worst in nearly three months of anti-government protests that have paralyzed Ukraine's capital in a struggle over the identity of a nation divided in loyalties between Russia and the West. It prompted the European Union to threaten sanctions against Ukrainian officials responsible for the violence and triggered angry rebukes from Moscow, which accused the West of triggering the clashes by backing the opposition.
Sanctions would typically include banning leading officials from traveling to the 28-nation bloc and - crucially - freezing their assets there. Travel bans and assets freezes for the powerful oligarchs who back President Viktor Yanukovych could prompt them to pressure him to change course.
But the bad blood runs so high that it's not clear whether an unstoppable force of conflict has been unleashed: The rising rage on both sides has fueled fears that the 46-million nation in the center of Europe could be sliding deeper into violence that could lead to its breakup. While most people in western regions of Ukraine resent Yanukovych, he still enjoys strong support in the mostly Russian-speaking eastern and southern regions, where many want strong ties with Russia.
Neither side now appears willing to compromise, with the opposition insisting on Yanukovych's resignation and early elections and the president prepared to fight till the end.
Radical protesters willing to confront police with violence were largely shunned at the start of the demonstrations three months ago, but they have become a key force in recent weeks, with moderate demonstrators bringing them food and some even preparing Molotov cocktails for them. Police also have turned increasingly brutal after law enforcement officers were killed.
The protests began in late November after Yanukovych turned away from a long-anticipated deal with the EU in exchange for a $15 billion bailout from Russia. The political maneuvering continued ever since, with both Moscow and the West eager to gain influence over this former Soviet republic.
The Kremlin said it put the next disbursement of its bailout on hold amid uncertainty over Ukraine's future and what it described as a "coup attempt."
Yanukovych on Wednesday blamed the protesters for the violence and said the opposition leaders "crossed a line when they called people to arms."
The European Union appears poised to impose sanctions as it called an extraordinary meeting of the 28-nation bloc's foreign ministers for Thursday.
European Commission President Jose Manuel Barroso called for "targeted measures against those responsible for violence and use of excessive force can be agreed ... as a matter of urgency."
"It is the political leadership of the country that has a responsibility to ensure the necessary protection of fundamental rights and freedoms," said Barroso, who heads the EU's executive arm. "It was with shock and utter dismay that we have been watching developments over the last 24 hours in Ukraine."
The latest bout of street violence began Tuesday when protesters attacked police lines and set fires outside parliament, accusing Yanukovych of ignoring their demands to enact constitutional reforms that would limit the president's power - a key opposition demand. Parliament, dominated by his supporters, was stalling on taking up a constitutional reform to limit presidential powers.
Police responded by attacking the protest camp. Armed with water cannons, stun grenades and rubber bullets, police dismantled some barricades and took part of the Maidan. But the protesters held their ground through the night, encircling the camp with new burning barricades of tires, furniture and debris.
On Wednesday morning, the center of Kiev was cordoned off by police, the subway was shut down and most shops on Kiev's main street were closed. But hundreds of Ukrainians still flocked to the opposition camp, some wearing balaclavas and armed with bats, others in everyday clothes and with makeup on, carrying food to protesters.
A group of young men and women poured petrol into plastic bottles, preparing fire bombs, while a volunteer walked past them distributing ham sandwiches from a tray. Another group of activists was busy crushing the pavement into pieces and into bags to fortify barricades.
"The revolution turned into a war with the authorities," said Vasyl Oleksenko, 57, a retired geologist from central Ukraine, who said he fled the night's violence fearing for his life, but returned to the square in the morning, feeling ashamed. "We must fight this bloody, criminal leadership. We must fight for our country, our Ukraine."
Yanukovych was defiant on Wednesday, his tone leaving little hope for a compromise.
"I again call on the leaders of the opposition ... to draw a boundary between themselves and radical forces which are provoking bloodshed and clashes with the security services," the president said in a statement. "If they don't want to leave (the square) - they should acknowledge that they are supporting radicals. Then the conversation with them will already be of a different kind." He also called a day of mourning for the dead on Thursday.
The Health Ministry said 25 people died in the clashes, some from gunshot wounds, and Kiev hospitals were struggling to treat hundreds of injured. Activists also set up a makeshift medical unit inside a landmark Orthodox Church not far from the camp, where volunteer medics were taking care of the wounded.
Meanwhile, in the western Ukrainian city of Lviv, where most residents yearn for stronger ties with the EU and have little sympathy for Yanukovych, protesters seized several government buildings, including the governor's office, police stations, prosecutors and security agency offices and the tax agency headquarters. They also broke into an Interior Ministry unit and set it on fire. The building was still smoldering Wednesday morning and some protesters were driving around town in police cars they had seized during the night.
Tensions continued mounting. The government imposed restrictions for transport moving toward Kiev, apparently to prevent more opposition activists from coming from the Western part of the country, and at least one train from Lviv was held outside Kiev. Several highways into Kiev were also blocked by police.
Acting Defense Minister Pavlo Lebedev told the ITAR-Tass news agency that he has dispatched a paratrooper brigade to Kiev to help protect arsenals. He refused to say if the unit could be used against protesters, the agency said.
Tensions soared after Russia said Monday that it was ready to resume providing the loans that Yanukovych's government needs to keep Ukraine's ailing economy afloat. This raised fears among the opposition that Yanukovych had made a deal with Moscow to stand firm against the protesters and would choose a Russian-leaning loyalist to be his new prime minister.
President Vladimir Putin's spokesman, Dmitry Peskov, said in a statement carried by Russian news agencies that Putin had a phone conversation with Yanukovych overnight. Peskov said that Putin hasn't given Yanukovych any advice how to settle the crisis, adding that it's up to the Ukrainian government.
Peskov also added that the next disbursement of a Russian bailout has remained on hold, saying the priority now is to settle the crisis, which he described as a "coup attempt."
The Russian Foreign Ministry issued a statement, blaming the West for the failure to condemn the opposition for the latest bout of violence.
EU leaders took the opposite stance, with Swedish Foreign Minister Carl Bildt putting the blame on Yanukovych in an unusually tough statement.
"Today, President Yanukovich has blood on his hands," Bildt said.
Svetlana Fedas in Lviv, Ukraine, Laura Mills and Vladimir Isachenkov in Moscow and John-Thor Dahlburg and Juergen Baetz in Brussels contributed to this report.