Flames engulfed Kiev's occupied Independence Square on Wednesday as police stormed the main protest camp in a bloody escalation of Ukraine's three-month political crisis which left at least 16 people dead.

Defiant President Viktor Yanukovych refused a chorus of calls from global leaders to halt the ferocious assault on the bloodiest day since protests broke out in November, when he ditched a pact with the EU in favour of closer ties with former Soviet master Russia.

The scene was apocalyptic as explosions echoed around central Kiev and the iconic square turned into a war zone.

Helmeted protesters hurling rocks, fireworks and Molotov cocktails fought back against black-armored riot squads targeting them with stun grenades and water cannon.

Smoke from burning tires billowed up into the icy night air, while laser lights, flags and the glare of several fires from burning tents flickered over the riled-up crowd.

"Yanukovych said that there is only one option... to clear Maidan and that everyone has to go home," opposition leader Vitali Klitschko said after a meeting with the president, using the local name for the square.

Several protesters ran or were stretchered out of their last stronghold, the imposing Trade Union House building on the square, as it went up in flames.

"If we hold on until morning, we have a chance to make it out," one protester said.

Kiev was essentially in lockdown as authorities halted the city's metro system and limited road traffic coming into the capital.

Police said seven officers died from gunshot wounds, while authorities and medics counted nine civilian deaths since Tuesday morning.

Another two civilians were found dead, but their bodies showed no exterior signs of violence, making it unclear how they had died.

More than 150 people were injured, including dozens of police officers, some with serious wounds.

As the turmoil intensified, US Vice President Joe Biden phoned Yanukovych to express his concern and urge him to "pull back government forces and to exercise maximum restraint," said a White House statement.

EU foreign policy chief Catherine Ashton said she was "deeply worried about the grave new escalation".

UN chief Ban Ki-moon called for restraint and dialogue, while NATO head Anders Fogh Rasmussen urged "all parties to refrain from violence and to urgently resume dialogue, including through the parliamentary process".

German Foreign Minister Frank-Walter Steinmeier warned that the violence could lead to EU sanctions against those responsible.

Sweden's Foreign Minister Carl Bildt tweeted: "Only person who can now stop catastrophe in Ukraine is President Yanukovich. His vacillation and violence responsible for situation."

However a top Russian lawmaker said Ukraine — which is divided between a pro-EU half and a pro-Russian half — was on the brink of a civil war that he claimed had been inflamed by the West.

"I consider that a significant amount of responsibility for this falls on the West and Western politicians, who are constantly putting pressure on the Ukrainian authorities," Alexei Pushkov told Interfax news agency.

The violence erupted after the crisis appeared to have abated in recent days, with both sides making concessions which saw protesters vacate Kiev city hall after being granted an amnesty deal.

Early on Tuesday, some 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 took back control of city hall and attacked Yanukovych's party headquarters with petrol bombs before smashing their way inside.

Yanukovych's ruling Regions Party said that an employee at its headquarters was found dead after the seizure of the building.

After threatening "grave actions" if the unrest did not cease, government sent in the riot police in the evening.

Police had warned women and children through loudspeakers to leave the square as they began their assault, which they described as "an anti-terrorist" operation.

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

Opposition leaders continued to address the crowd as the assault took place.

"This is a small island of freedom," the former boxing champion Klitschko said, declaring the protesters were "not going anywhere".

"The state has launched a war against its own people. Responsible democratic countries cannot stand back and let this happen."

Meanwhile, in the pro-EU western city of Lviv, some 5,000 protesters took control of an arms warehouse after setting fire to military buildings with Molotov cocktails.

Prosecutor-general Viktor Pshonka warned he would seek the "harshest punishments" for those deemed to have been behind Tuesday's violence.