Thousands of police have moved on a large protest camp in the centre of the Ukrainian capital, Kiev, BBC reported.
Police have torn down barricades close to the main opposition site on Independence Square.
A stand-off developed after clashes broke out between security forces and protesters.
The government's decision to withdraw from a free-trade deal with the EU last month has sparked huge street rallies by the opposition.
The latest police action comes after EU foreign policy chief Catherine Ashton held talks with President Viktor Yanukovych on Monday. She was cheered by crowds when she visited Kiev's main protest site.
Riot police and interior ministry officers dismantled some outer barriers but met resistance from opposition supporters.
There are unconfirmed reports that police have begun storming the Kiev city hall, seized by protesters just over a week ago.
The BBC's Steve Rosenberg in Kiev says a "sea of police" are surrounding Independence Square, seemingly waiting to move on the main camp there.
Protesters have been pouring water onto the icy ground to make it too slippery for the police to advance, our correspondent adds.
He says police have been pushing up against the barricades, while demonstrators are standing on top of the barriers.
Protesters shouted "Fascists" and "Kiev wake up".
Images showed people in the square singing Ukraine's national anthem.
A priest appealed to the riot police not to obey what he called "criminal orders", Ukraine's Hromadske.tv reported.
President Yanukovych said on Monday that government officials could visit Brussels this week to resume talks on the EU association agreement.
His statement came after hundreds of thousands of protesters turned out in Kiev on Sunday, demanding the resignation of the government within 48 hours.
Several people were hurt on Monday night as riot police advanced on protesters, dismantling a number of barricades in the centre of the capital.
Government buildings are blockaded with cars, barricades and tents.