* Opposition leaders meet Yanukovich for fresh talks
* Radicals clash with police overnight, then truce
* Other protesters allow only 'essential staff' into energy ministry
* Big anti-government rally expected over weekend
By Richard Balmforth and Jack Stubbs
KIEV, Jan 25 (Reuters) - Ukrainian opposition leaders held fresh talks with President Viktor Yanukovich on Saturday after overnight clashes between radical protesters and police, and an attempt by other activists to occupy the main energy ministry building.
Major rallies were expected to take place in the centre of Kiev later this weekend despite promises by Yanukovich to reshuffle the government and promote changes to sweeping anti-protest legislation.
A statement on the presidential website said the three opposition leaders - boxer Vitaly Klitschko, former economy minister Arseny Yatsenyuk and nationalist Oleh Tyahnibok - were in talks with Yanukovich on settling the two months of anti-government unrest.
But tension stayed high with Ukraine's interior minister saying that all those who stayed on Kiev's Independence Square - the crucible of the protest where hundreds camp overnight - and occupied public buildings would be considered by police to be "extremist groups".
Police would use force against those who went over to the side of the radical protesters, who have clashed with police in front of Dynamo Kiev football stadium since last Sunday, the minister, Vitaly Zakharcheno, said in a statement.
Adding to tension, Klitschko's Udar party tweeted that it believed a police order had been given already for police to storm the protest zones.
Radical protesters overnight lobbed petrol bombs, fireworks and other projectiles at police lines, despite the apparent concessions by Yanukovich, before a morning truce was called.
The overnight violence near Dynamo Kiev's stadium, the new flashpoint in unrest convulsing the former Soviet republic, left fires burning and smoke billowing over the area. Protesters kept up a drum-beat of sticks on corrugated metal.
Though the violence died out in early morning after a negotiated truce, about half a mile away (one kilometre away), protesters stormed into the energy ministry.
"There was an attempt to seize the building. About 100 people came, armed. I went to them and said that if they did not peacefully leave the building, then the whole energy system of Ukraine could collapse," Energy Minister Eduard Stavytsky told Reuters by telephone.
Stavytsky, who was shown on TV Fifth Channel angrily remonstrating with a black-helmeted activist, added: "What is taking place is a direct threat to the whole Ukrainian energy system."
A group of masked men wearing battle-fatigues and sticks maintained a blockade outside the building.
"We are here to check who goes in an out. We are allowing through only staff who are absolutely essential for the safe running of the ministry," one of them, 23-year-old Andriy, told Reuters.
Hundreds of activists have already occupied City Hall and the agricultural ministry, both close to the energy ministry building, in increasingly violent protests against Yanukovich's rule.
The rallies against Yanukovich erupted last November after he pulled out of a free trade deal with the European Union in favour of closer economic ties with Russia. They have since spiralled into protests against misrule and corruption.
Though the protest movement - known as the "EuroMaidan" - is largely peaceful, a hardcore of radicals have been fighting pitched battles with police away from the main protest on Independence Square.
Adding to tensions, the opposition raised the prospect of a state of emergency being declared and the interior minister admonished the opposition leaders for not reining in radical protesters.
"They can no longer control the radical elements who have occupied government buildings and are promoting violence," said Zakharchenko and he urged the international community not to "turn a blind eye' to what was taking place.
Overnight one policeman was shot in the head and three more were kidnapped on Independence Square, the statement said. But the interior ministry later said that at least two of those held had been released after the intervention of peaceful demonstrators and diplomats.
The United States has warned Yanukovich that his failure to ease the standoff could have "consequences" for its relationship with Ukraine. Germany, France and other Western governments have also urged him to talk to the opposition.
Russia on Saturday stepped up its warnings against international interference in Ukraine, telling European Union officials to prevent outside meddling and cautioning the United States against inflammatory statements.
"I told (U.S. Secretary of State) John Kerry that is very important now not to interfere in the process and to avoid any statements that will only heat up the situation," said Foreign Minister Sergei Lavrov.
"I hoped he heard me," he said, in an interview with Vesti v Subbotu state television news programme.
The EU's point man on Ukraine, Stefan Fule, who was in Kiev on Friday and met Yanukovich as well as opposition leaders, said his talks "showed the need for a series of concrete steps to first start to rebuild trust of people by stopping the spiral of violence and intimidation".
EU foreign policy chief Catherine Ashton is scheduled to visit Kiev next week.
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