Western diplomats urged Ukrainian authorities to respect the massive protests gripping the country against the government's decision to freeze ties with the EU and turn to Moscow instead.
Several thousand activists kept up the demonstrations at a central square in the capital Kiev and besieged government meetings as the Organization for Security and Cooperation in Europe's ministerial council began its meeting on the other side of the river.
The meeting had been scheduled long before the protests that have been dominating the country.
US Assistant Secretary of State Victoria Nuland challenged Ukrainian authorities to meet the protests constructively.
"This is Ukraine's moment to meet the aspirations of its people or disappoint them," she told the OSCE meeting. "Democratic norms and the rule of law must be upheld."
Britain's Minister for Europe David Liddington called on authorities to respect the right of citizens to "peacefully assemble."
"The eyes of the world are on Ukraine today," he said.
Prime Minister Mykola Azarov tried to put a positive spin on the tense situation, saying the protests "are a completely normal development in a country where democracy is developing."
"We will do everything we can to ensure this is a peaceful protest," Azarov said.
With President Viktor Yanukovych away in China, the government showed no sign of yielding to the protests. Police have promised not to use force, but law enforcement bodies were detaining and investigating scores of opposition activists.
Azarov has also warned several pro-EU western cities in Ukraine which have gone on a strike that the central government in Kiev might cut off funding to them.
The demonstrators were sparked by Yanukovych's decision to ditch a significant treaty with the 28-nation European Union after strong pressure from Russia.
They were also galvanized by riot police's violent breakup of a small, peaceful rally last month.
The protesters are demanding that the government resign and that early elections be called.
Azarov chided the demonstrators, who have occupied or blocked government buildings, saying they are contradicting the values that they claim to support.
"That is not the European way forward," Azarov said of the building occupations.
He also said Ukraine remains committed to moving forward with the EU association and characterized Yanukovych's shelving of the signing as only a pause.
"The timeout we have taken is clearly taken only because of economic difficulties," he said.
Ukraine says it cannot absorb the trade losses with Russia it likely would suffer if it had signed the EU agreement last month.
Meanwhile, Russian Foreign Minister Sergey Lavrov, speaking directly after Nuland at the conference, notably did not mention the Ukrainian protests, which have had a strong anti-Russian element.
Russian President Vladimir Putin's government is determined to bring Ukraine, a former Soviet republic, back into its area of influence.