* Truce called after night of violence
* Protesters allow only 'essential staff' into energy ministry
* Big anti-government rally expected over weekend
* Interior minister says opposition does not control radicals
By Richard Balmforth and Jack Stubbs
KIEV, Jan 25 (Reuters) - Anti-government protesters called an uneasy truce in the Ukrainian capital Kiev on Saturday after violent overnight clashes with police, but a separate group made an attempt to take over the main energy ministry building.
Radical protesters overnight lobbed petrol bombs, fireworks and other projectiles at police lines, despite apparent concessions by President Viktor Yanukovich to the opposition.
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.
The violence near Dynamo Kiev football stadium, the new flashpoint in two months of 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.
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.
Tension remained high with the opposition raising the prospect of a state of emergency being declared and the interior minister admonishing 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 Interior Minister Vitaly Zakharchenko in a statement after the overnight violence.
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.
Security forces believe they are being held in the occupied City Hall and have demanded their immediate release.
In a joint statement, the three main opposition leaders, including boxer-turned-politician Vitaly Klitschko added to tension ahead of an expected big rally on Sunday, saying they had information the Yanukovich leadership was preparing to introduce a state of emergency.
"The centre of national resistance has reliable information that the presidential administration has prepared a decision to introduce a state of emergency and 'clear out' the Maidan," they said in a statement.
Yanukovich's signature on such a decree would be authorisation for "the shooting of hundreds of people, the beginning of war by the authorities against their own people".
The United States has warned Yanukovich his failure to de-escalate 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.