* Yanukovich's main rival in jail as vote goes ahead
* His Regions party likely to secure slim majority
* But faces new opposition force led by popular boxer
* Observers will pronounce on whether vote free and fair
By Richard Balmforth and Olzhas Auyezov
KIEV (Reuters) - Ukrainians voted on Sunday in an election that President Viktor Yanukovich's pro-business ruling party seemed likely to win, but it may now face a re-energized opposition which has promised to fight growing authoritarianism and corruption.
With Yanukovich's main rival, Yulia Tymoshenko, in jail and with the West seeing the poll as a test of Ukraine's commitment to democracy, interest will focus on the judgment that international monitors will hand down on Monday.
The former Soviet republic of 46 million is more isolated internationally than it has been for years. Tymoshenko's continued imprisonment has put it at odds with the United States and the European Union, while Russia turns a deaf ear to Kiev's calls for cheaper gas.
At home, the government's popularity has been hit by tax and pensions policies and a failure to stamp out corruption, prompting it to shy away from painful reforms that could secure much-needed IMF lending to shore up its export-driven economy.
Despite this and growing apathy among an electorate tired of political bickering, opinion polls have shown Yanukovich's Party of the Regions leading the joint opposition, which includes Tymoshenko's Batkivshchyna (Fatherland) party, and a liberal party headed by boxing champion Vitaly Klitschko.
Commentators expect Regions, bankrolled by industrialists and drawing on state resources, to keep a majority in the 450-seat assembly with support possibly from communists and some independents.
"I have voted for stability, for the country's economic development, for the improvement of living standards," Yanukovich told reporters as he cast his ballot in Kiev.
Even if it wins, Regions faces a tougher time in parliament.
Klitschko, the WBC world heavyweight champion, who heads the UDAR (Punch) party, says he will team up with the opposition led by former economy minister Arseny Yatsenyuk to fight corruption, which they say deters entrepreneurial spirit and foreign investment.
From her jail in Kharkiv in Ukraine's northeast, Tymoshenko issued a statement that Yanukovich, who comes up for re-election in 2015, would set up a "dictatorship and never again give up power by peaceful means".
Tymoshenko was jailed for seven years last year for abuse of office relating to a 2009 gas deal with Russia which she made when she was prime minister. The Yanukovich government says the agreement saddled Ukraine with an enormous price for gas supplies.
"I am voting for my mother's freedom, for freedom to political prisoners, for justice and so that we do not wake up behind barbed wire tomorrow," Tymoshenko's daughter Yevgenia said at the polling station.
Voters' frustration with both the current and the previous cabinets plays into the hands of newcomer Klitschko, who on Sunday urged voters to "vote as your heart tells you".
"I voted for UDAR as it is a new force," said Valentyn, 45, as he walked out of a polling station in Kiev. "I am sick of the old ones. Something needs to be changed."
"We have seen some parties in power and others as well," said Tetyana, 27, referring to Batkivshchyna and the Regions. "We have seen the results."
Even in Donetsk, Yanukovich's main stronghold in the east of the country, many voters said they were disillusioned by the government's record.
"I voted for the Regions Party but simply because it is the lesser of the evils. I can't say I am a great fan of the Regions, but all the rest are worse," said 58-year-old Viktor Grigoryev, a head of section in the construction sector.
"They (the Regions) have the experience of working in posts of responsibility and have proven they can do things," he added.
Viktoriya, aged 45, who works in the state housing sector, said she had also voted for the Regions and applauded the development during the June Euro-2012 soccer championship.
"They built an airport in Donetsk, carried out the Euro football here. They added to my Mum's pension. All the 'orange' people used to do was talk but do nothing," she said referring to previous governments of the jailed Tymoshenko.
RUSSIAN LANGUAGE PROMISE
The government raised public sector wages and pensions ahead of the vote, recovering some of its lost support at the cost of widening the budget deficit which tripled year-on-year to $2 billion for the period of January to August. Ukraine's economy is vulnerable to falling demand for steel and other exports.
The Regions has also promised to make Russian an official state language alongside Ukrainian - a move aimed at winning back disenchanted supporters in Russian-speaking areas of the east and south but which alienates many voters elsewhere.
Polling stations opened at 8 a.m. and were to close at 8 p.m (2 p.m. EDT) with exit polls following swiftly afterwards.
Of the 450 seats in the single-chamber parliament, 225 will be filled by voters casting ballots for parties to send candidates from a list.
The other half will be decided by voting for individual candidates on a first-past-the-post basis - a feature re-introduced by the Regions which is assumed to favor the party.
Though results will begin to trickle in almost immediately, an accurate overall picture will emerge only much later on Monday since counts in individual constituencies take longer.
International observers from the OSCE European security and human rights body are due to give their judgment on Monday on how fair and free they perceived the poll to have been.
A positive assessment could improve Yanukovich's image before Ukraine takes over the organization's chair in January.
(Additional reporting by Olzhas Auyezov; Writing by Richard Balmforth; Editing by Giles Elgood)