* Security forces set deadline to end disturbances
* Opposition want president to cede control of government
* Fresh clashes take place after Russia pledges more aid (Adds fresh violence, fires on edge of Kiev square, Fuele)
By Pavel Polityuk and Richard Balmforth
KIEV, Feb 18 (Reuters) - Ukrainian protesters hurled petrol bombs, fireworks and stones at riot police on the edge of Kiev's Independence Square on Tuesday, and nine people died in the worst day of violence since demonstrations erupted against President Viktor Yanukovich.
Western powers warned Yanukovich against trying to smash the 12 week-old 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 two policemen had died in Tuesday's clashes.
Earlier the State Security Service (SBU), in a joint statement with the interior ministry, had set protesters a 6 p.m. (1600 GMT) deadline to end street disorder or face "tough measures".
Forces loyal to the Russian-backed president broke through front-line barricades near the Dynamo Kiev soccer stadium and advanced to the edge of the occupied Independence Square. They were met after nightfall with a hail of petrol bombs and fireworks, responding with bursts of water cannon.
Fires lit by protesters raged on the fringes of the square to prevent police advancing as opposition speakers harangued the crowd, interspersed with patriotic music.
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 former Soviet republic's heavily indebted economy.
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 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.
Clashes raged for several hours earlier 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 said the two officers and three protesters died of gunshot wounds. Two more protesters suffered heart attacks while one died in a fire and another in a traffic accident.
The State Security Service and interior ministry had threatened tough action. "If by 6 p.m. the disturbances have not ended, we will be obliged to restore order by all means envisaged by law," their statement said.
The defence ministry issued a separate warning to protesters to evacuate the officers' club near parliament.
Klitschko, a former world heavyweight boxing champion who leads one of three main opposition groups, told protesters on the occupied square: "We cannot exclude the possibility of use of force in an assault on the Maidan."
Right Sector, a militant far-right group, added to tension by calling on people holding weapons to go to Independence Square, centre of the revolt, to protect it from a possible offensive by 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 telephoned his Ukrainian counterpart to warn against sliding back into violence and keep working for a political solution.
Monday's $2 billion cash injection, a resumption of the $15 billion aid package, was seen as a signal that Russia believes Yanukovich has a plan to end the protests and has 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.
"I think Russia received some kind of assurances from the Kiev leadership that were satisfactory, because only a day before there was nothing like it," said Gleb Pavlovsky, former Kremlin adviser and political analyst in Moscow.
"I think Yanukovich showed he would stick firmly by his position in talks (with the opposition), he would not make excessive concessions, he would fight the radicals who are getting stronger in the opposition ... and that the (new) prime minister would not be a member of the opposition."
Yet rather than boosting Yanukovich, Moscow's move may have helped to provoke a more violent turn in the protests, especially from those demonstrators who have a strong anti-Kremlin agenda.
Several thousand protesters torched vehicles and hurled stones. Police replied by firing rubber bullets and stun and smoke grenades from trucks and from the tops of buildings, forcing the protesters back by about 100 metres.
"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, an opposition deputy.
Inside parliament, opposition leaders brought proceedings to a halt by blocking the speaker's tribune and opposition leader Klitschko urged Yanukovich to take riot police off the streets to avert further "conflict in society".
The protesters had marched to parliament to back the opposition leaders' calls for Yanukovich to relinquish what they call his "dictatorial" powers and particularly his control of the economy and the security forces. (additional reporting by Marcin Goettig in Kiev, 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)