Ukraine opposition issues ultimatum
Ukrainian opposition leaders have issued an ultimatum to President Viktor Yanukovych, after talks failed to resolve the political stalemate.
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Vitali Klitschko said he would lead pro-EU protesters "on the attack" in the capital, Kiev, if the government refused to call snap elections.
Prime Minister Mykola Azarov said compromises "might be possible", but the opposition should avoid ultimatums.
Two activists were killed in clashes with police in Kiev on Wednesday.
Prosecutors confirmed they had died from gunshot wounds.
They were the first fatalities since the anti-government protests flared up in late November over Mr Yanukovych's decision to pull out of a landmark treaty with the EU.
Late on Wednesday, Ukraine's Radio Liberty reported the death of a third activist.
The body of Yuri Verbitsky was found in a forest outside Kiev, bearing signs of torture, according to the broadcaster.
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He had reportedly been abducted earlier this week with activist Ihor Lutsenko, who was later released. Mr Lutsenko is said to be in hospital.
The BBC's Duncan Crawford has spoken to a student, Mikhail Nizkoguz, who accused riot police of dragging him from the street on Monday and torturing him by beating and stabbing.
His face and body is covered with cuts and bruises.
He said police accused him of firing fireworks at them - but that he was only taking pictures.
The BBC has asked police to respond to his allegations but has had no response.
Hundreds of people have been injured in the clashes, though some of the violence has been blamed on a little-known far-right group, Right Sector.
Wednesday's unrest came on the day that new anti-protest laws entered into force. Parliament approved the laws last week, triggering renewed protests which spilled into violence on Sunday night
The centre of Kiev remains extremely tense, the BBC's Daniel Sandford reports.
As dawn broke on Thursday, the barricades were still burning, billowing black smoke from the piles of tyres that now mark the front line between the riot police and the protestors.
Speaking at a mass rally on Wednesday evening, Mr Klitschko said the president could end the stand-off "without bloodshed" by calling early elections, but that "tomorrow, if the president does not respond... then we will go on the attack", to roars of approval from the crowd.
Mr Klitschko said police were preparing to clear demonstrators out of the main protest encampment at Maidan (or Independence Square).
"We must do all we can to stop them clearing us out," he told demonstrators.
Another opposition leader, Arseniy Yatseniuk, said the government had 24 hours to respond to the demands, which also include the lifting of the new anti-protest laws.
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"If this does not happen, we will march forward together. If it's a bullet to the head, then it's a bullet to the head," he declared.
But the prime minister said opposition leaders should be "more humble".