Tens of thousands of demonstrators gathered in Ukraine's capital, Kyiv, to protest the government's decision not sign a landmark trade deal with the European Union.
Protesters waved Ukrainian and EU flags while chanting pro-Western slogans as they headed to the city's European Square.
Officially, between 20- and 50-thousand people are estimated to have turned out for the rally, which was organized by the country's opposition. Activists have put the number at above 100,000.
Thousands of riot police were deployed across the city with some clashes reported after demonstrators attempted to storm a government building.
Elsewhere in Kyiv, groups of pro-government demonstrators marched in support of the government's decision to halt the signing of the EU agreement.
Ukraine's Cabinet unexpectedly announced last week it was suspending preparations to sign a EU trade and cooperation agreement.
They announced instead that Ukraine would resume an "active dialogue" with Belarus and Kazakhstan, members of a Russian-led customs union, and other former Soviet republics with the goal of reviving trade and economic relations.
The country's parliament earlier refused to pass legislation that would free jailed former prime minister Yulia Tymoshenko, which was a key EU condition for signing the EU deal.