The ex-Soviet nation of 46 million has been lurching through its worst political crisis since the 2004 pro-democracy Orange Revolution as it once again debates its exact role as a geopolitical bridge between Russia and western states.

President Viktor Yanukovych’s abrupt U-turn last month on a deal with Brussels that could have pulled Kiev out of Moscow’s orbit has exposed deep cultural fissures that have been running between Ukraine’s nationalist west and more Russified east for most of its post-Soviet history.

Opposition leaders — openly backed, to Russia’s dismay, by top US and EU officials — are demanding the government’s immediate dismissal and snap elections that could break Yanukovych’s dominance over politics.

Both conditions have been rejected by despite signs that some in the government are willing to open direct talks.

The Kremlin has also accused the West of aggressively interfering in the international affairs of a sovereign nation that has chosen to remain on good terms with Moscow.

EU governments have hit back at Russia with charges of using threats of potential trade sanctions against Ukraine as a form of blackmail that would assure Kiev’s continued reliance on Moscow.

Yanukovych courts Russia

The size of Sunday’s protest threatens to eclipse earlier rallies in Kiev and western Ukraine that brought several hundred thousand out on the streets on December 1.

Those demonstrations were fed in part by anger over a violent police crackdown the day before on a few hundred protesters who were occupying Kiev’s icon Independence Square — the heart of the 2004 revolt.

But now the opposition is outraged at news that Yanukovych had paid a snap visit to Russia for talks with President Vladimir Putin about a new strategic partnership that could indefinitely delay talk of an EU pact.

“There has to be more than a million of us here (on Sunday),” boxing champion turned opposition leader Vitali Klitschko told supporters in Independence Square late Saturday.

“Our future depends on each one of you.”

Yanukovych’s office said his meeting Friday with Putin — their fourth in just over a month — focused on “trade and economic cooperation … and preparation for the future treaty on strategic partnership”.

But some reports said the two also talked about Russia providing Ukraine with more than $12 billion dollars in loans and cheap gas that would make sure the economically struggling country does not look for Western help in the years to come.

The Economist’s senior editor Edward Lucas — known for writing books critical of the Kremlin — tweeted that his sources said Yanukovych had in return assured Putin of Ukraine’s participation in a Russian-led Customs Union starting in 2015.

Putin hopes to build up the union into an economic powerhouse that includes most of the former Soviet nations and rivals the European Union in importance.

Both the Ukrainian president’s office and the Kremlin denied the reports.

But rumours of a possible deal spread quickly across Kiev and prompted opposition leaders to warn of grave consequences if they proved to be true.

“Sunday’s protest can result in a very tragic ending for our country if (Yanukovych) really did sell out Ukraine,” parliament’s Batkivshchyna (Fatherland) opposition party leader Arseniy Yatsenyuk said Saturday.