Ukrainian opposition leaders have issued an ultimatum to President Viktor Yanukovych, after talks failed to resolve the political stalemate.

Vitali Klitschko said he would lead pro-EU protesters "on the attack" in the capital, Kiev, if the government refused to call snap elections.

Speaking at a mass rally on Wednesday evening, 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.

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.

"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.

Prime Minister Mykola Azarov said compromises "might be possible", but the opposition should avoid ultimatums.

Three activists were killed in clashes with police in Kiev on Wednesday, while hundreds have been injured.