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* Worst street violence since late January
* Opposition want president to cede control of government
* Fresh clashes take place after Russia pledges more aid
By Pavel Polityuk and Richard Balmforth
KIEV, Feb 18 (Reuters) - Several thousand anti-government protesters clashed with police near Ukraine's parliament on Tuesday, torching vehicles and hurling stones in the worst violence to rock the capital Kiev in more than three weeks.
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 renewed clashes piled more pressure on President Viktor Yanukovich to strike a deal with the opposition to reduce his presidential powers and defuse a 12-week crisis that has turned Kiev city centre into a fortified camp of resistance.
Inside parliament, where opposition leaders brought proceedings to a halt by blocking the speaker's tribune, opposition leader Vitaly Klitschko urged Yanukovich to take riot police off the streets to avert further "conflict in society".
"I am appealing to the president. Take the Berkut (riot police) and interior forces off the streets. Do this and it will provide a way out. It will be the decision of a real man," the boxer-turned-politician told reporters inside parliament.
"We are not talking any more about hours but about minutes."
The protesters had marched to the parliament building to press 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.
But when they were blocked by a line of trucks about 100 metres from the building, they hurled stones at police, a Reuters witness said, and set three trucks ablaze with petrol bombs. Ukrainian television said five protesters had been hurt in the clashes. The report could not immediately be confirmed.
As the clashes extended into early afternoon, protesters ransacked a nearby office of Yanukovich's Party of the Regions.
Yanukovich has been battling the opposition-led street protests since he walked away from a trade pact with the European Union in November and opted instead for forging closer economic ties with Russia, Ukraine's Soviet-era master.
The protests have since broadened into a revolt against perceived sleaze and corruption in the Yanukovich leadership.
"WE'RE NOT FOR SALE"
Opposition leaders are pressing him to accept curbs on his powers that would allow them to form an independent government to end the street unrest, now in its third month, and to save the economy from collapse.
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 recovery, while Russia accuses them of meddling.
Yanukovich, who must soon name his choice for a new prime minister, got a boost on Monday when Moscow said it would give Kiev a fresh cash injection this week of $2 billion - part of a $15 billion aid package to shore up a heavily indebted economy.
News of the fresh credit from Russia failed to cheer the currency market, where the troubled Ukrainian hryvnia fell by up to 1.6 percent against the dollar on Tuesday, Reuters trading system showed.
It also failed to impress the protesters.
"If all the money that today's authorities have stolen was returned we'd be able to help Russia with our money," said 35-year-old Serhiy Vashko. "We don't need their money."
"We don't need this money from Russia because it is not meant to help but to buy us. But we are not for sale. Can't they see that this is simply a dirty bribe?," said Valentin Sypko.
Thousands of protesters have turned Kiev city centre and its main Independence Square into a fortified camp, surrounded by barricades of tyres, sandbags and barbed wire. They say they will not withdraw until Yanukovich has made real concessions.
At least six people have died in the unrest, though there had been no violent clashes between radicals and riot police for more than three weeks until Tuesday's fresh outbreak. (Writing By Richard Balmforth; Editing by Gareth Jones)