The opposition has called for a vast turnout Sunday. Rallies on the previous two Sundays drew hundreds of thousands of protesters. That same day, Yanukovych's Party of Regions has called for a pro-government demonstration that it claims will bring 200,000 people to Kiev.
The prospect of a huge cadre of government-backers in the vicinity of protesters has raised fears of provocations that would induce riot police to crack down brutally. The opposition blames provocateurs allegedly planted by the government for a previous violent demonstration dispersal.
"We are really afraid, and we know that there will be very many provocations," world superheavyweight boxing champion and opposition leader Vitali Klitschko told The Associated Press.
The U.S. said it was disappointed that discussions between the government and opposition were apparently unproductive. Washington also called for calm.
"It is absolutely imperative that this weekend's protests be allowed to proceed peacefully," U.S. State Department deputy spokeswoman Marie Harf said. "So we'll certainly be watching what happens over the next few days."
Officials have said there will be no action taken against peaceful protests, but those assurances are shadowed by the pre-dawn events of Wednesday, when thousands of riot police converged on Independence Square, where a large protesters' tent camp has been set up and demonstrators gather around the clock.
The police tore down some barricades and tents, but demonstrators stood their ground and police left after sunrise. A smaller police contingent scuffled with demonstrators who have occupied the nearby city hall, but also retreated.
It remained unclear whether the police left because of the protesters' resistance, or if they had been ordered only to try to intimidate the demonstrators rather than drive them out. Some opposition figures suggest the stand-down showed Yanukovych was losing the loyalty of the country's extensive security forces.