More than 100,000 people in the Ukrainian capital Kiev on Sunday protested against a move by the government to delay an association deal with the EU.
The protest is said to be the largest since the 2004 Orange Revolution, which overturned a rigged presidential poll.
The BBC reports police fired tear gas as protesters tried to break through a cordon around government buildings.
A pro-government rally a few miles away attracted about 10,000 people.
Several rallies have been held over the last few days in Kiev and other cities, but the BBC reports Sunday's was the largest so far.
Ukraine made the decision on the EU deal last week, saying it could not afford to break ties with Russia, which is trying to bring Kiev into its own customs union with two other former Soviet republics, Belarus and Kazakhstan.
President Vladimir Putin has accused the EU of blackmailing Ukraine to sign the deal during a summit in Lithuania next week.
A free trade agreement with the EU would mean Ukraine adopting different trade rules.
Ukraine depends on imports of Russian gas, but recently Gazprom complained that Ukraine had fallen behind in payments.
In 2009 Russia cut off gas supplies to Ukraine in a dispute about prices, causing fuel shortages across Europe in the middle of winter.
On Friday, Prime Minister Mykola Azarov said the decision not to sign the EU deal was motivated solely by economics and was "tactical". He said it did not alter Ukraine's overall development strategy.
The government says it is now looking into setting up a joint commission to promote ties between Ukraine, Russia and the EU.