KIEV, Ukraine — Thick, dark smoke rose above the center of the Ukrainian capital amid the boom of police stun grenades Wednesday, as officers in riot gear sought to push demonstrators away from the city’s main square following deadly clashes between police and protesters that left at least 25 people dead and hundreds injured and raised fears of a civil war.

After several hours of relative calm, confrontation flared up again Wednesday afternoon, with hundreds of police amassing on the edges of Independence Square, known as the Maidan, throwing stun grenades and using water cannons in a bid to disperse protesters. Thousands of activists armed with fire bombs and rocks held their ground, defending the square which has been a bastion and symbol for the demonstrators.

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, and the worst in the country’s post-Soviet history. 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.

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.”

President Viktor 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.”

Sanctions would at first typically include banning leading officials from traveling to the 28-nation bloc and freezing their assets there.

“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,” he added.

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.