By Pavel Polityuk and Marcin Goettig
KIEV (Reuters) - Ukrainian riot police charged protesters occupying a central Kiev square early on Wednesday after the bloodiest day since the former Soviet republic, caught in a geopolitical struggle between Russia and the West, won its independence.
Police battled their way into Independence Square - centre of three months of protests against President Viktor Yanukovich - but demonstrators, some armed with clubs and wearing helmets and body armour, tried to stand their ground.
Smoke billowed from burning tents and piles of tyres and wood as thousands of protesters held on to the centre of the square, a Reuters cameraman said. Several floors of a trade union building, used as an anti-government headquarters, were on fire.
At least 18 people, including seven policemen, died on Tuesday during hours of violence between security forces and civilians. Many were killed by gunshot and hundreds more injured, with dozens of them in a serious condition, police and opposition representatives said.
Alarmed Western governments demanded restraint and dialogue. U.S. Vice President Joe Biden called Yanukovich, urging him to pull back the government forces and to exercise maximum restraint, the White House said.
Earlier, the state security service set a deadline for the demonstrators to end disorder or face "tough measures". Then the police advanced up to the square before launching a full assault in the early hours, throwing stun grenades.
Nationwide demonstrations erupted in November after Yanukovich bowed to Russian pressure and pulled out of a planned far-reaching trade agreement with the European Union, deciding instead to accept a Kremlin bailout for the heavily indebted economy.
Western powers warned Yanukovich against trying to smash the pro-European demonstrations, urging him to turn back to Europe and the prospect of an IMF-supported economic recovery, while Russia accused them of meddling.
Ukraine has been rocked periodically by political turmoil since gaining independence from the Soviet Union more than 22 years ago, but it has never experienced violence on this scale.
As the security forces moved forward, opposition leader Vitaly Klitschko reacted defiantly, telling supporters on the square: "We will not leave here. This is an island of freedom. We will defend it."
The world champion boxer-turned politician later arrived at Yanukovich's office for talks, Klitschko's spokeswoman said, but he and another opposition leader, former economy minister Arseny Yatsenyuk, were still waiting to see the president an hour later.
Earlier on Tuesday the State Security Service (SBU), in a joint statement with the interior ministry, signalled the government's intentions. "If by 6 p.m. the disturbances have not ended, we will be obliged to restore order by all means envisaged by law," they said.
The riot police moved in hours after Moscow gave Ukraine $2 billion in aid for its crippled economy which it had been holding back to demand decisive action to crush the protests.
PRAYING FOR UKRAINE
EU Enlargement Commissioner Stefan Fuele said he had spoken to Ukraine's acting prime minister, who had given assurances that the authorities would try to avoid using live firearms.
"For the sake of the Ukrainians and for the sake of the future of that country, I will pray that he is right," Fuele told a public event in Brussels.
A police spokeswoman gave a variety of reasons for the deaths including gunshot wounds, a traffic accident and heart attacks. One protester died in a fire.
Right Sector, a militant far-right group, added to tensions by calling on people holding weapons to go to Independence Square - also known as Maidan - to protect it from the security forces.
As protesters and police battled on the streets of Kiev, Russia called the escalation a "direct result of connivance by Western politicians and European structures that have shut their eyes ... to the aggressive actions of radical forces".
EU foreign policy chief Catherine Ashton, who has tried to broker a power-sharing transition, urge Ukraine's leadership "to address the root causes of the crisis".
Germany's foreign minister, Frank-Walter Steinmeier, telephoned his Ukrainian counterpart to warn against sliding back into violence and to urge the government to keep working for a political solution.
"News of a fresh escalation of violence is alarming. We are shocked to hear of the dead and injured today," Steinmeier said in a statement, raising the possibility of EU sanctions against Ukrainian leaders.
"Those responsible for taking any decisions that lead to the further spilling of blood must know that the reserve Europe has shown in terms of personal sanctions will be reconsidered," he added.
Monday's $2 billion cash injection, a resumption of a $15 billion aid package, was seen as a signal that Russia believed Yanukovich had a plan to end the protests and had dropped any idea of bringing opposition leaders into government.
While Russian President Vladimir Putin seems to have won the battle for influence in Ukraine for now, protesters who have occupied the centre of the capital are not going quietly.
(additional reporting by Marcin Goettig in Kiev, Alexandra Hudson in Berlin, Steve Holland in Washington, Elizabeth Piper in Moscow and Adrian Croft in Brussels; writing by Richard Balmforth, Paul Taylor and David Stamp; editing by Robin Pomeroy)