Anti-government protesters use fireworks during clashes with Interior Ministry members and riot police in Independence Square in central Kiev. Picture: Olga Yakimovich

Kiev -

Anti-government protesters in Kiev braced on Wednesday for a fresh assault by riot police after a day of fierce clashes left at least 25 people dead in Ukraine's worst crisis since the collapse of the Soviet Union.

The European Union said it would consider sanctions against those behind the deadly unrest, as embattled President Viktor Yanukovych blamed the opposition for the biggest escalation of violence yet in three months of protests.

Security forces stormed the capital's Independence Square overnight, bringing down a volley of tear gas and stun grenades on thousands of demonstrators as authorities sought to reclaim the city's main protest zone.

Demonstrators responded by throwing Molotov cocktails and burning tyres.

The square's sprawling tent camp was a scene of devastation early on Wednesday as riot police temporarily halted their assault.

Smoke billowed into the morning sky as lines of riot officers faced off against protesters - both sides clutching shields and wearing helmets and body armour, and divided by a barricade of fire.

The surge in violence, in a country torn between a future allied to the West and to Russia, sparked alarm in Europe and the United States.

EU foreign policy chief Catherine Ashton summoned the bloc's foreign ministers for emergency talks on Ukraine on Thursday, with sanctions being considered.

“All possible options will be explored, including restrictive measures against those responsible for repression and human rights violations,” she said.

Calls for sanctions were echoed by French President Francois Hollande and Poland's Prime Minister Donald Tusk, while US Vice President Joe Biden urged Yanukovych in a phone call to “pull back government forces and to exercise maximum restraint”.

Germany blamed the worsening crisis on Yanukovych's refusal to engage in serious dialogue with the leaders of the protests, which first erupted in November after the Ukrainian leader rejected an EU pact in favour of closer ties with Moscow.

In a televised address to the nation as clashes raged, a defiant Yanukovych accused the opposition of undemocratic behaviour.

“The leaders of the opposition... have crossed the limits by calling for people to take up arms,” he said.

Russia, which offered debt-laden Ukraine a $15 billion bailout after Yanukovych spurned the EU pact and has repeatedly condemned Western interference in the Ukrainian crisis, described the protests as “an attempted coup d'etat”.

Late night talks between Yanukovych and the opposition failed to go anywhere, prompting opposition leaders to call on protesters to stand their ground on the Maidan, as Independence Square is known.

“This is a small island of freedom,” opposition leader and former boxing champion Vitali Klitschko said after the talks, adding that the protesters were “not going anywhere”.

Kiev was effectively in lockdown as authorities shut down the city's metro system and limited traffic into the capital. Schools in the centre were closed.

Ukraine's health ministry said 25 people had died in the clashes since Tuesday morning and another 241 were in hospital. The dead include 10 policemen.

A journalist working for a pro-government Kiev newspaper was among the dead after he was shot by masked men, his newspaper Vesti said.

Yanukovych called for a day of mourning on Thursday.

The latest outbreak of the violence surprised many, as tensions appeared to have been subsiding in recent days with both sides making concessions that saw protesters vacate Kiev city hall on Sunday after being granted an amnesty deal.

But on Tuesday, about 20 000 anti-Yanukovych protesters clashed with police outside parliament as they rallied for lawmakers to strip the president of a raft of powers.

Running street battles broke out and protesters re-occupied city hall and attacked Yanukovych's party headquarters with petrol bombs.

After threatening “grave actions” if the unrest did not cease, police descended on the square in the evening, warning women and children through loudspeakers to leave as they began their “anti-terrorist” operation.

But about 25 000 people, many of them wearing makeshift body protection and wielding iron bars and bats, remained to battle the riot squads.

The violence also spread to the west of the country, where thousands of protesters overran public buildings, including the police and special forces' headquarters in the historic city of Lviv, where they took control of an arms warehouse.

In Ternopil, protesters lobbed Molotov cocktails at a local government building before occupying it, while similar scenes played out in the city of Ivano-Frankivsk. - Sapa-AFP