* Security forces set deadline to end disturbances
* Opposition want president to cede control of government
* Fresh clashes take place after Russia pledges more aid (Releads, updates death toll)
By Pavel Polityuk and Richard Balmforth
KIEV, Feb 18 (Reuters) - Ukrainian riot police advanced on Tuesday onto a central Kiev square occupied by protesters, after at least 14 people died in the worst day of violence since demonstrations erupted against President Viktor Yanukovich 12 weeks ago.
Protesters on Independence Square responded with petrol bombs, fireworks and stones, as police moved slowly forward, hours after the state security service had set a deadline for the demonstrators to end disorder or face "tough measures".
Live television footage showed the police throwing stun grenades at the protesters separated from them by a line of burning tents, tyres and wood.
Western powers warned Yanukovich against trying to smash the pro-European demonstrations and opposition leader Vitaly Klitschko, fearing an assault, urged women and children to leave the central square - also known as Maidan - "to avoid further victims".
A police spokeswoman said seven civilians and six policemen died in several hours of clashes, some from gunshot wounds, in what was also Ukraine's bloodiest day since it gained independence from the Soviet Union in 1991. A representative of the protesters said one other civilian had also died.
Earlier 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 which it had been holding back to demand decisive action to crush the protests.
Nationwide protests against Yanukovich erupted in November after he 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.
In what has become a geo-political tussle redolent of the Cold War, the United States and its Western allies are urging Yanukovich to turn back to Europe and the prospect of an IMF-supported economic recovery, while Russia accuses them of meddling.
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.
In Washington, White House spokesman Jay Carney said the United States was appalled by the violence and urged Yanukovich to restart a dialogue with opposition leaders. "Force will not resolve the crisis," Carney told a briefing.
Earlier on Tuesday, clashes raged for several hours outside the parliament building, where opposition lawmaker Lesya Orobets said three demonstrators were killed and taken to a nearby officers' club used as a medical centre. More than 100 people were injured, she said.
"Three bodies of our supporters are in the building. Another seven are close to dying (because of wounds)," she said on her Facebook page. Two more bodies were lying in front of a Metro station on the southeastern side of the square, a photographer told Reuters.
The 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, centre of the revolt, to protect it from 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".
NATO Secretary-General Anders Fogh Rasmussen called on all parties to refrain from violence. 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.
Later he raised the possibility of 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 said in a statement.
Monday's $2 billion cash injection, a resumption of the $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.
In another apparent gesture towards Moscow, a Ukrainian government source said state gas company Naftogaz has paid back $1.3 billion of its 2013 debt to Russian gas monopoly Gazprom , although it still owes $1.5 billion.
Ukraine's hryvnia currency fell towards five-year lows after the fresh outbreak of violence, with importers clamouring for dollars.
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.
"The authorities do not want to compromise on any issue ... We understand that yet another odious candidate will be put forward (for prime minister), one who will be unable to restore the economy or end the political crisis," said Vyacheslav Kyrylenko, a Ukrainian opposition deputy. (additional reporting by Marcin Goettig in Kiev, Alexandra Hudson in Berlin, Eric Beech in Washington, Elizabeth Piper in Moscow, Adrian Croft in Brussels and Anna Wlodarczak-Semczuk in Warsaw; writing by Richard Balmforth and Paul Taylor; editing by Jeremy Gaunt and David Stamp)