The Ukrainian presidential election held Sunday was headed to a run-off in three weeks between Prime Minister Viktor Yanukovych and leading opposition figure Viktor Yushchenko after neither of the two major candidates in a field of more than 20 received a majority necessary to declare victory, according to incomplete returns and exit polls.

With two-thirds of the votes counted early Monday, Yanukovych had 43 percent of the vote to 36 percent for Yushchenko, according to the Central Elections Commission.

Conflicting exit polls had given the lead to both candidates. Official final results were not expected until later in the day.

Yushchenko, a former prime minister who supports further integration with the West, led with 44.4 percent of the vote, according to one poll in which respondents marked a secret ballot without revealing their choice to the pollster. Yanukovych, who is backed by President Leonid Kuchma and Russian leaders, had 38 percent.

In another survey conducted by the same consortium of agencies, the result was reversed, with Yanukovych leading Yushchenko by 42.7 percent to 38.3 percent. In that survey, a different set of voters was interviewed face to face about their choice.

"We think that in the face-to-face interviews people didn't wish to say the truth," said Ilko Kucheriv, director of the Democratic Initiatives Foundation, a nongovernmental organization that oversaw the conflicting polls. "We think the secret ballot poll is more reliable."

A third poll by a Russian agency gave the prime minister a 4-point lead.

Nearly 75 percent of registered voters turned out, according to the Central Elections Commission.

Following a bitter and competitive race marred by violence and charges of dirty tricks, election day was also marked by accusations of vote-rigging from the two leading candidates' camps. The differing exit poll results is likely to add to the sense of turmoil marking the race.

Tens of thousands of troops and police were deployed across the country, and the Interior Ministry said it would use force to put down any attempt to disrupt the process.

Observers sponsored by Russia and other countries are separately monitoring the election and will issue reports Monday.

The two candidates face another three weeks of campaigning in a head-to-head contest that will be decided Nov. 21. Opinion polls suggest that support for minor party politicians will split fairly evenly between the two leading candidates, ensuring a tight and tense race to the finish.

Yanukovych, who was all but endorsed by Russian President Vladimir Putin during an official visit last week, has advocated closer ties with Russia but has also appealed to the pocketbooks of Ukrainians by emphasizing the country's strong economic growth in the last year. Yanukovych nearly doubled pensions and other social payments in September, a move that cut into Yushchenko's significant early lead, according to opinion polls.

"Today the government did everything for the peaceful conduct of the vote,'' Yanukovych, 54, said after casting his ballot.

Yushchenko, a reformer who wants Ukraine to integrate rapidly with the European Union as well as consider NATO membership, had attacked the current government as corrupt and increasingly authoritarian.

"I want to live in a proper country where people respect honest leaders and do not fear them . . . where there is rule of law and an honest legal system,'' said Yushchenko, 50, speaking to reporters at a polling station.

Voters were highly polarized by the campaigns, and many people approached outside polling stations declined to be interviewed or refused to give their family name to a reporter.

"When we got our independence, we thought we were free and now we're going back to the same type of control we had under the Communists," said Alexander Siparenko, 52, who works at a science institute in Kiev. "There is a lot of fear."

But another voter, Ivan Petrov, 50, said Yushchenko threatened Ukrainian independence because he was too close to the West, particularly to the United States.

"Yanukovych is our man," he said. "He's one of us, closer to us."

Ukrainians cast their ballots in a village near Kiev, the capital. Election day was marked by accusations of vote-rigging from the two leading candidates' camps.Opposition candidate Viktor Yushchenko supports further integration with West.Prime Minister Viktor Yanukovych is backed by current President Leonid Kuchma and Russia.