Tension mounted in Kiev as authorities planned a rally of their own, bussing in thousands of Yanukovych supporters from the provinces, setting up a camp in a park not far from the opposition and vowing a "non-stop protest".
Ukraine's security services were on high alert, as opposition leaders urged a massive turnout for the rally on Independence Square where the crowd swelled to hundreds of thousands last weekend.
Protesters have been camped out on the square for more than three weeks since Yanukovych failed to sign the EU deal.
The Ukrainian leader is under intense pressure to decide whether to align his nation with the West by signing a deal with the EU, or to join a Moscow-led Customs Union.
He is set to travel to Moscow next week to meet President Vladimir Putin. The opposition movement adamantly opposes a proposed free trade deal with Russia.
"All Ukrainians must go out to Maidan to voice their aspirations to live in a modern European country," said Vitali Klitschko, a former heavyweight champion and head of the UDAR party, using Independence Square's Ukrainian name.
"We see the attempts to frighten us, but they will fail."
The party of jailed opposition leader Yulia Tymoshenko said on its website that Sunday's rally, which starts at noon (10:00 GMT) would feature "leaders of opposition parties, public figures and foreign guests, particularly American senators."
Outspoken Republican Senator John McCain flew into Kiev on Saturday and held meetings with Klitschko, nationalist leader Oleg Tyagnybok and the head of Tymoshenko's party, Arseniy Yatsenyuk.
Late Saturday he posted a picture on Twitter of tens of thousands packing Independence Square for a performance by one of the country's top bands, Okean Elzy, dedicated to injured protesters.
The senator praised what he called "an incredible show of patriotism."
Yanukovych on Saturday made a move to appease the protest movement by suspending Kiev's mayor and the deputy head of its security council over a brutal crackdown on protesters' opposition camp on November 30.
Opposition leaders praised the decision but said it did not go far enough, calling for the dismissal of the interior minister and Prime Minister Mykola Azarov.
Thousands of Yanukovych supporters arrived Saturday for a rally on the city's Europe Square and set up a camp in the Mariinsky park with braziers, a field kitchen and dozens of large army tents.
Some participants said they were to sleep in their buses amid freezing temperatures.
A top lawmaker with the ruling Regions Party, Mykhailo Chechetov said Yanukovych's supporters were there to stay.
"Our peaceful rally is non-stop, it will continue until the situation in the country stabilises," Chechetov told the Saturday rally.
Azarov gave a speech at the rally condemning opposition leaders and saying that "their place is at the garbage dump."
Police used buses to block the road and prevent opposition supporters from interrupting the rally which was held close to one of the barricades built by the protesters.