* At least 14 protesters, seven policemen killed
* Protesters lose ground, their headquarters ablaze
* Biden calls Yanukovich to urge police pull-back
* Opposition talks with president break down
* Russia criticizes "connivance by Western politicians" (Adds Yanukovich statement)
By Pavel Polityuk and Marcin Goettig
KIEV, Feb 19 (Reuters) - Ukrainian riot police fought 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.
After hours of clashes, police were gaining ground in Independence Square, centre of three months of protests against President Viktor Yanukovich, but demonstrators managed to find protection behind a burning barricade of tires and wood.
Shrouded in plumes of black smoke, police were trying to put out the fire with two water cannons and protesters responded by hurling petrol bombs at the police vehicles, a Reuters cameraman said.
Police have gained control of almost half the square and several floors of a trade union building, used as an anti-government headquarters, were on fire as dawn was breaking.
At least 14 protesters and seven policemen were killed in violence that erupted on Tuesday and continued into the early hours of Wednesday. Many were killed by gunshot and hundreds of people were injured, with dozens in serious condition, police and opposition representatives said.
Representatives of protesters said one journalist was shot dead.
Alarmed Western governments demanded restraint and dialogue. U.S. Vice President Joe Biden called Yanukovich, urging him to pull back government forces and exercise maximum restraint, the White House said.
Opposition leaders Vitaly Klitschko and Arseny Yatsenyuk said that they had quit talks with President Viktor Yanukovich without reaching any agreement on how to end the violence.
"The government must immediately withdraw troops and put an end to the bloody conflict, because people are dying. I told Yanukovich this," Klitschko said after the late night talks. "How can we hold talks while blood is being shed?"
Yanukovich later urged leaders of the opposition to dissociate themselves from radicals, saying otherwise he would "talk differently" with them.
The unrest has spread to at least three cities in the western part of the country. Police said protesters had seized regional administration headquarters in the cities of Ivano-Frankivsk and Lviv. Media said protesters torched the main police station in the city of Ternopil.
The authorities have restricted traffic coming into the capital to prevent protesters from getting reinforcements. Kiev authorities also closed down the underground.
Earlier, the state security service set a deadline for the demonstrators to end disorder or face "tough measures". Then the police advanced to the square before launching a full assault in the early hours, throwing stun grenades and using water cannons.
Nationwide demonstrations erupted in November after Yanukovich bowed to Russian pressure and pulled out of a planned far-reaching trade pact 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 the EU 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.
"We are now facing of one of the most dramatic episodes in Ukrainian history," opposition leader Yatsenyuk said in a video message after emergency talks with the president failed.
As the security forces moved forward, Klitschko, a former world champion boxer, reacted defiantly, telling supporters on the square: "We will not leave here. This is an island of freedom. We will defend it."
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 that 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 tension by urging people holding weapons to go to Independence Square, also known as Maidan, to protect it from the security forces.
As protesters and police battled, 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".
U.N. Secretary-General Ban Ki-moon called for restraint, said his spokesman, Martin Nesirky, adding: "He is extremely concerned over today's reports of renewed violence and fatalities."
EU foreign policy chief Catherine Ashton, who has tried to broker a power-sharing transition, urged 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."
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, the protesters are not going quietly. (Additional reporting by Alexandra Hudson in Berlin, Steve Holland in Washington, Elizabeth Piper in Moscow and Adrian Croft in Brussels; Writing by David Stamp; Editing by Mohammad Zargham, Ken Wills and Robert Birsel)