Opposition leaders said an olive branch proposal from President Viktor Yanukovych was not enough to end the ex-Soviet country's worst crisis since independence and demanded snap elections this year.
The protests began more than two months ago over Yanukovych's rejection of a pact with the European Union under Russian pressure. But they have now turned into an all-out bid to oust him from power.
Protesters have already attempted to blockade 14 of the 25 regional administrations, including in southern and eastern parts of the nation of 46 million that predominantly speak Russian and share an historical allegiance to Moscow.
Justice Minister Olena Lukash, who is taking part in negotiations between the opposition and Yanukovych, said she would ask for the talks to be broken off if the building was not freed.
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"I will be forced to ask the president of Ukraine to stop the talks if the building is not freed immediately and negotiators are not given a chance to find a peaceful solution to the conflict," Lukash told Ukraine's Inter channel.
Europe has urged dialogue between the two sides -- a call echoed by Pope Francis, who voiced hope in his weekly Angelus prayer that "the search for common good may prevail in the hearts of all".
Under unprecedented pressure, Yanukovych on Saturday offered the opposition posts in government including that of prime minister, but his opponents said the offer fell short of their needs.
Yanukovych proposed sharing leadership with Fatherland party chief Arseniy Yatsenyuk as prime minister and UDAR (Punch) chief and former boxing champion Vitali Klitschko as deputy prime minister.