Vote-rigging led to the 2004 presidential election result being annulled and the current president, Viktor Yushchenko, sweeping to power after the 'Orange Revolution.'
 
Polls opened at 7am (04:00 GMT) and were due to close at 10pm (1900 GMT) and official results are due on Monday morning.
 
No clear winner is expected and protracted coalition talks to form a government are almost certain.
 
Laurence Lee, Al Jazeera's correspondent in Donetsk, said allegations of vote rigging were coming in from other parts of the country.
 
"There is potential for this to go to the courts and that could take week and weeks to resolve."
 
'Total lawlessness'
 
Ukranians have faced three years of political paralysis as pro-Western Yushchenko has sought to push his Russian-backed rival from power.
 
Viktor Yanukovych, the prime minister, was forecast to win the most votes with his Regions Party.
 
He said Ukraine was "being put to the test," and warned he would not recognise the results in the case of "total lawlessness" in the poll.
 
But Yushchenko hopes an alliance with Yulia Tymoshenko, his partner in the "Orange Revolution," will enable him to form a  coalition government and remove Yanukovych from power.
 
"I am convinced that today the nation will choose change,"  Yushchenko said after voting in central Kiev.
 
Political resurrection
 
Yushchenko defeated Yanukovych in a re-run of the presidential election in 2004 after the original result was declared invalid following a series of mass pro-democracy protests that became known as the Orange Revolution.
 
Since then however, Yushenko has failed to implement many of the reforms he promised and Yanukovych, who favours closer ties with Russia, has undergone a political resurrection.

After being accused of rigging the presidential vote in 2004, he became prime minister after his party won a majority in elections in 2006.

Yushchenko is pinning his hopes on a last-minute alliance with Tymoshenko - a partnership that could give their parties control of parliament, even if Yanukovych's bloc wins the most votes on Sunday.

Increased fears

Forging a government coalition with Tymoshenko, however, could mean weeks of negotiation between their two parties, and Yanukovych has signalled that he would not give up power easily - threatening protracted election uncertainty.

Yanukovych, who draws his support from Ukraine's Russian-speaking east and south, fiercely resisted Yushchenko's decision to dissolve parliament in April and call new elections after the president accused him of seeking to seize power.

Yanukovych eventually agreed to the vote on Sunday, but has suggested victory would be the only outcome he is prepared to accept.

Ukraine has 37.5 million registered voters but irregularities in voter registration has increased fears there could be legal challenges to any result.