At least 200,000 pro-European demonstrators began a mass rally in the Ukrainian capital Sunday in a fresh show of force against President Viktor Yanukovych after his failure to sign a key EU agreement.

Opposition parties have called on "all Ukrainians" to mass on Independence Square, where demonstrators angered by Yanukovych's failure to sign the EU pact have held rolling protests for over three weeks.

Outspoken United States Senator John McCain met Ukraine opposition leaders including former heavyweight boxing champion Vitali Klitschko in the capital Kiev on Saturday ahead of a mass rally.

McCain met Klitschko, the head of the UDAR party, as well as nationalist leader Oleg Tyagnybok and the leader of the party of jailed Yulia Tymoshenko, Arseniy Yatsenyuk.

A counter-rally in support of Ukrainian President Viktor Yanukovych massed in Kiev on Saturday, a short distance from pro-EU opposition protesters set up behind barricades in the capital.

Police spokeswoman Olga Bilyk put the number of pro-Yanukovych demonstrators in the city's Europe Square at 60,000.

Republican Senator John McCain will head to Ukraine's capital for the weekend ahead of fresh protests planned by opposition activists, his office said Friday.

"Senator McCain is traveling to Kiev to meet with government officials, opposition leaders and civil society as they work to determine their country's future," McCain spokesman Brian Rogers said.

Ukrainian President Viktor Yanukovych on Sunday suspended his deputy security council chief and Kiev's mayor after prosecutors alleged they pressured police into using force against pro-EU protesters.

"The president of Ukraine, Viktor Yanukovych, suspended the deputy chief of the national security council, Volodymyr Sivkovich, and mayor Olexander Popov over their suspected involvement in a violation of rights" of protesters in Kiev on the night of November 29-30, a presidential statement said.

Russian Foreign Minister Sergei Lavrov on Saturday said that Western countries appear to have lost "their sense of reality" over the pro-EU protests in Ukraine, which he called the work of provocateurs.

He said the "frenzied" reaction to Ukraine's scrapping of the Association Agreement with the European Union last month was a disproportionate response to a "perfectly normal" decision.

Ukraine's opposition Friday sat down for a tense round of talks with President Viktor Yanukovych for the first time since mass protests broke out three weeks ago but said he had failed to meet their demands.

The talks came amid mounting tensions ahead of what could be a potentially explosive weekend in Kiev, with both the opposition and supporters of the ruling Regions Party vowing to hold mass rallies in the city center.

Ukraine's opposition called Thursday for a mass weekend rally, as pro-European demonstrators seek to increase the pressure on President Viktor Yanukovych following a failed police raid.

"We invite everyone to come to our rally at midday on Sunday," said opposition leader Arseniy Yatsenyuk, as protestors on Kiev's iconic Independence Square marked three weeks of demonstrations over the government's decision to reject a historic EU deal.

Ukraine is still welcome but not obliged to work with Russia on its post-Soviet Customs Union, seen as a potential counterweight to the European Union, President Vladimir Putin said on Thursday.

"We are not imposing anything on anyone, but if our (Ukrainian) friends want joint work (on the Customs Union) we are ready for a continuation of that work at expert level," Putin said in his annual address to the nation as pro-EU protests raged in Ukraine.

Ukrainian demonstrators celebrated holding three weeks of protests Thursday over the government's decision to reject a historic EU deal, as the United States threatened sanctions after a failed police raid on the protest barricades.

Tensions in the capital eased after a showdown in the early hours of Wednesday, when riot police tried to drive the protest camp out of the iconic Independence Square in the ex-Soviet country's deepest political crisis in a decade.