Thousands of Ukrainian protesters are keeping up their vigil in Kyiv, where western diplomats have gathered for a meeting of the Organization for Security and Cooperation in Europe.
Demonstrators have been calling for the ouster of President Viktor Yanukovych, after he backed out of a trade deal with the European Union.
On Thursday, several western officials attending the OSCE meeting challenged the Ukrainian government to consider the demands of protesters.
U.S. Assistant Secretary of State Victoria Nuland attended the meeting and said this was "Ukraine's moment to meet the aspirations of its people" or to disappoint them.
Russian Foreign Minister Sergei Lavrov denounced Western criticism of Ukraine's handling of the demonstrations.
Ukrainian Prime Minister Mykola Azarov told the group his government was "ready for dialogue" with the protesters.
Yanukovych is visiting China.
On Wednesday, three former Ukrainian heads of state - Leonid Kravchuk, Leonid Kuchma and Viktor Yushchenko - issued a joint statement, endorsing the anti-government protests.
The protests erupted last week, after the Yanukovych government refused to sign the EU trade deal, announcing instead that it would seek to preserve and strengthen its economic lifeline with Moscow.
Russia - Ukraine's largest foreign investor, trading partner and chief natural gas supplier - has in recent months exerted strong economic pressure on cash-strapped Kyiv in an apparent effort to derail the EU deal.
Earlier this year, Moscow imposed restrictions on goods from Ukraine, forcing a 25-percent drop in Ukrainian exports and dragging the Ukrainian economy into recession.