Buoyed by swelling anti-government protests, opposition leaders in Ukraine pressed for more concessions from President Viktor Yanukovych before accepting his power-sharing proposal.

As protests spread from the capital Kiev to provinces, Mr. Yanukovych offered the post of Prime Minister to one of the opposition leaders, Arseniy Yatsenyuk, and agreed to share his sweeping constitutional powers with the Parliament and the government.

Emerging from late-night talks with the President on Saturday, opposition leaders told supporters that Mr. Yanukovych’s offer fell short of their demands and vowed to press for early presidential elections.

“We are demanding presidential elections to be held this year and we won’t back down,” said former boxing champion-turned politician Vitali Klitschko. The vote is not due until 2015.

The opposition leaders said they would press for signing a free trade agreement with the European Union and freeing political prisoners, above all former Prime Minister Yulia Tymoshenko. It was Mr. Yanukovych’s refusal to sign the E.U. pact that triggered protests more than two months ago.

Protesters applauded their leaders’ intransigence and went on to storm a building in central Kiev where about 200 police had camped out. After several hours of pelting the building with Molotov cocktails and stones, the attackers forced the police to leave the building.

Protests have engulfed a large swathe of Ukraine, with government offices stormed in most pro-European western and central province. With protests gaining momentum, the opposition feels increasingly confident and is reluctant to walk into what it sees as a trap Mr. Yanukovych set up with his offer of top government jobs.

“They’ve been robbing Ukraine for the past three-and-a-half years, the coffers are empty, and now they want us to assume responsibility for the country,” Mr. Yatsenyuk said.

A Russian government source warned last week that a power change in Ukraine could prompt Moscow to revise the terms of a $20-billion bailout package it agreed to give Ukraine after Mr. Yanukovych spurned the pact with Europe. A special session of Parliament called by Mr. Yanukovych’s majority party, Regions of Ukraine, on Tuesday will show how far the President is prepared to go to appease the opposition.

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