The European Union suspended historic partnership talks with Ukraine on Sunday as 200,000 protesters massed in the heart of Kiev demanding that the government recommit itself to the West.
The ex-Soviet nation of 46 million has been at the heart of a furious diplomatic tug of war since President Viktor Yanukovych's shock decision last month to ditch a landmark EU association agreement and seek closer ties with old master Russia.
EU Enlargement Commissioner Stefan Fuele announced in a surprise tweet that the 28-nation bloc was halting all negotiations until it received "a clear commitment" from Yanukovych that Ukraine was serious about the deal.
"Ukraine: Words and deeds of President and government regarding #AssocAgreement further and further apart," said Fuele.
Yanukovych is due in Moscow on Tuesday for talks with Russian President Vladimir Putin that protesters occupying central Kiev's iconic Independence Square fear could result in an even firmer alliance between the two neighbours.
Demonstrators have planned another huge rally to coincide with Yanukovych's meeting with Putin on Tuesday evening. They were warmly reassured of continued US backing on Sunday by Republican Senator John McCain -- one of Washington's staunchest critics of Kremlin rule.
"To all Ukraine, America stands with you," McCain called out to a cheering sea of people who chanted "Thank you!" in English in return.
Protesters were eagerly awaiting McCain's appearance amid freezing weather when news spread that EU enlargement chief Fuele had said Brussels was halting talks with Kiev after an inconclusive Friday meeting.
EU officials told Ukraine that further discussions required a "clear commitment (to) sign (but) Work on hold, had no answer," said Fuele.
A Ukrainian government spokesman quickly responded that Kiev remained serious about the negotiations and did not treat Fuele's tweet as the formal position of the bloc.
Prime Minister Mykola Azarov has requested a 20-billion-euro ($27.5-billion) loan from the European Union before it signs the closer trade and political association deal.
Ukraine says the money would compensate for the losses it would suffer from the trade sanctions threatened by Russia -- which has promised cheaper natural gas shipments should Kiev join a customs union championed by Putin instead.
EU officials have rejected the loan request and argued that Ukraine stood to benefit over the long term from the removal of EU trade barriers.
Swedish Foreign Minister Carl Bildt -- a strong proponent of Ukraine's integration who has paid repeated visits to Kiev -- said on Sunday that the original terms of the EU deal stood.
"The door is wide open for Ukraine to sign association and free trade agreement with EU," Bildt tweeted. "It's ready. Any time."
Ukraine's security services were on high alert on Sunday as around 5,000 Yanukovych supporters bussed in from the provinces began what they said would be "non-stop protests" in a park near Independence Square.
"If the president made a mistake, that doesn't mean you need to gather in the Maidan (Independence Square)," said a pensioner named Galina Beresneva as she stood amid dozens of large army tents and a field kitchen set up by pro-government organisers.
Yanukovych moved to appease the protest movement on Saturday by suspending Kiev's mayor and the deputy head of his security council over a violent November 30 crackdown protesters.
But opposition leaders said they were also seeking the dismissal of the interior minister and the prime minister.
McCain began his visit to Kiev on Saturday by holding meetings with the troika of opposition leaders who most prominently include boxing champ turned UDAR (Punch) party leader Vitaly Klitschko.
The powerful US senator declared from a stage from which opposition leaders continually address the crowd that Ukraine's protests -- the largest since the 2004 pro-democracy Orange Revolution -- were "inspiring" the entire world.
"Ukraine will make Europe better and Europe will make Ukraine better," said McCain.
His comments came as nearly 500 Parisians rallied in support of Kiev's EU aspirations near the Eiffel Tower while several hundred more waved EU and Ukrainian flags outside London's parliament building.
A 56-year-old electrician named Sergei, one of the demonstrators in Independence Square, said he felt encouraged by McCain's talk.
"Of course it's important to have the support of the American parliament and president," said Sergei. "In Ukraine we don't think the US wishes us harm."