Hundreds of thousands of Ukrainians rallied in the centre of Kiev in the biggest protest since 2004's pro-democracy Orange Revolution, denouncing the decision of the country's president to turn away from Europe.
Angry crowds that brandished the flag of the nationalist Freedom Party toppled the statue of Russian revolutionary leader Vladimir Lenin, police said, protesting the country's strengthening ties with Moscow.
Demonstrators hope to force President Viktor Yanukovich to resign after he refused to sign a historic trade pact with European Union leaders under pressure from Russia.
"Yanukovich took a decision to join the club of dictators," read a statement from jailed opposition leader Yulia Tymoshenko.
"We must peacefully and legally oust him from power.
"He is no longer the president of our state, he is a tyrant who must answer for every drop of blood that has been shed," she added.
Protesters on Sunday were calling for hard hats to be donated to them, after previous mass demonstrations were broken up by riot police - resulting in dozens of injuries.
A call to the streets
"It's really quite remarkable," Al Jazeera's Robin Forestier-Walker, reporting from Kiev earlier on Sunday, said.
"It looks like organisers have been able to get large numbers out, and the rally here has only just begun."
Yanukovich's decision to drop political and free trade agreements with the EU in favour of tighter Russian ties and a crackdown last week on protesters plunged the ex-Soviet nation into its worst political crisis in a decade.
The president on Friday incensed the opposition and its supporters further by reportedly discussing a strategic partnership treaty with Russian President Vladimir Putin, who wants Ukraine to join a Moscow-led Customs Union.
"The Customs Union is another Soviet Union. We've already been there," said protester Olexander Kovalenko.
Boxing champion-turned-opposition leader Vitali Klitschko had said a million people should take to the streets of Kiev on Sunday, even amid the snow and bitter cold.
But Putin slammed the protests, saying they looked more "like a pogrom than a revolution".