The nationalist west is backing the pro-Western opposition leader, Viktor Yushchenko, to become president, while the Russian-speaking east supports the disputed victory of Prime Minister Viktor Yanukovich.
An independent state for only 13 years, this strategic former Soviet republic of 48 million people has long been divided between its European-minded west that borders Hungary, Slovakia and Poland, and the east neighbouring Russia.
But the conflict over last weekend's presidential poll has led to a sudden polarisation that opposition leader Viktor Yushchenko warned put it "on the verge of civil conflict".
As tens of thousands of his supporters massed in Kiev's central square for the fourth day on Thursday, the key western region of Lviv, an opposition bastion, said it would only take orders from Yushchenko.
"We have a legitimately elected president, Viktor Yushchenko," said Mikhail Sendak, the speaker of the Lviv regional assembly, which on Tuesday had voted to recognise the opposition leader as president.
"We in Lviv region will only obey his orders. I call on other regions to follow suit and to recognise Yushchenko. The bandits falsified the results of the elections," he said.
Other parts of the west, where Yushchenko got up to 90% of the vote, have rejected the official results and proclaimed Yushchenko president.
But in the eastern coal-mining region of Donetsk, where Yanukovich hails from, supporters of the prime minister travelled to the capital to voice their support.
Viktor Yushchenko (R) says the
prime minster stole the election
Igor Yankovsky, a miner from Donetsk in his 40s, was among a group of 200 coal workers who gathered in a park opposite the government administration building.
"We came here to support our president because he is our local man. His government raised pensions for us," he said.
In a sign of the tension, the miner said that when his group went to buy bread in a shop in Kiev - where residents are largely pro-opposition - the shopkeeper refused to sell them anything and turned her back on them.
"We want to be friends with other ex-Soviet republics, Russia, Belarus. I heard Yushchenko saying that people from the east are second-class citizens," Yankovsky said.
The densely-populated east, where Russian is spoken, is the industrial heartland of the country, with a huge coal-mining sector and heavy industry as well as a military-industrial complex that dates back to Soviet times.
Western Ukraine was historically part of Poland before being absorbed by Soviet Ukraine after World War II, and has a long history of resistance, notably with partisan units fighting Soviet rule until the 1950s.
"I am afraid that the government will use every means at its disposal to hang onto power"
Ukraine's former deputy defense minister
And this region was at the forefront of the movement that led to the independence of Ukraine in 1991.
Religion is also a big dividing factor, with Uniate Catholics in the west and Orthodox believers in the east.
Ukraine's former deputy defense minister Vadim Grechannikov said he feared a conflict in Ukraine, where the centre including the capital Kiev is mainly pro-opposition and the south including Russian-speaking Crimea backs Yanukovich.
"I am afraid that the government will use every means at its disposal to hang onto power," he said.
But "many people in the military command are pro-opposition," Grechannikov added, "especially in the regions where Yushchenko won.
It is possible that he could take power in part of the country and assume control over state structures, including the army."
In a significant statement, the military command in western Ukraine said on Thursday it was staying out of the political crisis gripping the country and would not act against its own people.
Viktor Yanukovich was declared
the winner of the elections
Mikhail Pogrebinski, a Ukrainian analyst closely linked with Kuchma, said the ball was in the court of the "revolutionaries" backing Yushchenko.
He said the pro-Russian communities in east Ukraine "will not recognise a candidate placed in power with the help of the European Union".
Without a power-sharing compromise, he warned, "it's a civil war".