Viktor Yushchenko, the leading opposition candidate for the presidency of Ukraine, planned to hold a rally on Tuesday in a central square in this small city. But over the weekend, city officials suddenly moved a traveling zoo and amusement park onto the square, forcing Yushchenko's backers to scurry for another location.

"They will try anything to harass us, even tigers and bears and zebras, but it won't work," said Irina Gerashchenko, a spokeswoman for the candidate, as she scanned a crowd of about 15,000 people who showed up Tuesday at a square away from the city center.

On Sunday, Ukraine will hold elections following an intensely competitive campaign marred by dirty tricks, bombings and harassment on a scale unusual even by the standards of post-Soviet electioneering.

The opposition also faces accusations of rough tactics, including inciting violence aimed at intimidating electoral officials. But perhaps the most controversial claim is that government agents poisoned Yushchenko last summer. He disappeared from view for five weeks, reemerging to campaign with his formerly youthful face disfigured. The government denies the accusation.

Torn by the pulls of neighboring Russia and the West, this country of 48 million people has swung between emerging democracy and authoritarianism in the 13 years since it gained independence with the collapse of the Soviet Union. In the past year, it has strengthened military ties with the United States, sending 1,650 troops to the multinational force in Iraq despite public skepticism.

There are 26 candidates running, but the race is effectively between two men: the ruling party candidate, Viktor Yanukovych, 54, who is backed by Russia and favors closer ties, and Yushchenko, 50, a former prime minister, who champions rapid integration with the West, notably the European Union.

"This election is not about left or right," said Vadim Karasev, director of the Institute for Global Strategies, a political research group. "It's a historic struggle between the past and the future. Whoever wins is going to consolidate the character of the country's development for a long time."

Yanukovych is the handpicked successor of the current president, Leonid Kuchma, whose 10 years in power have been shadowed by accusations of political killings and the enrichment of associates, including his son-in-law, through questionable privatizations of key industries.

Orphaned at age 5, Yanukovych had a troubled youth and was convicted separately for robbery and assault. He was later pardoned, and after gaining a degree in mechanical engineering, he rose to become governor of Donetsk, a city at the center of Ukraine's metal and mining industries.

"I came from a very poor family, and my main dream in life was to break out of this poverty," Yanukovych told the Ukrainian media recently. He declined to be interviewed for this article.

Yanukovych was appointed prime minister in 2002 and began this campaign badly trailing Yushchenko. But he has steadily risen in the polls following his decision last month to nearly double all pensions in the country. "People see what they have in their pockets and on their tables," said Sergei Tigipko, Yanukovych's campaign manager.

"A month ago it looked like a catastrophe," said one campaign consultant who works for the prime minister. "Now I think he can win, and win without falsifying the vote."

The two candidates are now neck-and-neck with Yanukovych slightly ahead, according to opinion polls commissioned by groups sympathetic to each camp.

The prime minister has clearly benefited from his almost complete domination of airtime on state-controlled television networks. Broadcast journalists, speaking anonymously, said they receive almost daily orders telling them what to cover and how to cover it.

Analysts say Yanukovych has also been helped by an economy that grew 13 percent this year, due mostly to expanded steel exports. But many parts of Ukraine remain mired in poverty, and incomes fall far short of those in neighboring countries, including Russia, where millions of Ukrainians work, often illegally.

Yanukovych has signaled his intent to build much closer ties with Russia. He supports making Russian, which is widely spoken in eastern Ukraine, the country's second official language. He wants to reach an agreement with Russia to allow Ukrainians to take Russian citizenship. And he has endorsed the creation of an economic union between Russia, Ukraine, Belarus and Khazakstan, a key goal of Russian President Vladimir Putin.

At the same time, he says he wants cordial if cautious relations with the West. "We will be heading to the E.U. gradually," he said at a recent rally.

Dozens of Russian political operatives have come to Ukraine to support his campaign. And Putin, in Kiev this week for a three-day state visit, praised Yanukovych.

"This is not just a question of high growth rates" in the economy, Putin said in a live broadcast carried on three national networks during his visit. "The government led by Viktor Yanukovych has gone further and has ensured that this is high-quality growth. . . . This is something very valuable, and it is a very good example."

The opposition has smarted at what it perceives to be intervention by Putin against them -- a fear that one Kremlin-paid consultant in Ukraine said in an interview was not misplaced. The Russian government, said Sergei Markov, director of the Institute of Political Studies in Moscow, is "too closely connected" to the Yanukovych campaign. "The Kremlin regards the imposition of Yushchenko as an aggression against Russia."

Tigipko said Russian consultants had only a marginal role and said he did not know Markov. Yushchenko's campaign employs a number of American consultants in mid-level positions. U.S. government development funds have for years helped underwrite grass-roots organizations that political analysts say tend to support the opposition.

In an interview, Yushchenko responded diplomatically to Putin's visit. "I don't think it's the best time for him to visit," he said.

Effectively barred from television, Yushchenko has been barnstorming the country. "I am happy I am not an American politician. I am happy I am not a Russian politician," he told the crowd in Kirovograd. "I am happy to be a Ukrainian politician."

A former banker, backed by both pro-Western reformers and nationalists from the Ukrainian-speaking west of the country, Yushchenko has called for rapid integration with the West, including joining the E.U. and cooperating with NATO, with a view to possibly joining it. Yanukovych is against NATO membership for Ukraine.

Yushchenko, like Yanukovych, has said Ukrainian troops should be withdrawn from Iraq. The war has drawn little support here, according to opinion polls.

"My opponent says he is taking Ukraine to the East and this is the only possible direction for Ukraine," Yushchenko said. "Russia is our everlasting neighbor and a strategic partner for Ukraine. But we also border the European Union, and it would be a huge mistake to not hitch ourselves to the train of European integration and common security and defense arrangements."

Yushchenko, whose wife is an American of Ukrainian origin, has faced accusations from opponents of being an American stooge married to a CIA agent.

Leaflets and T-shirts allegedly produced by Yanukovych supporters variously depict the United States as a bloodsucker and show President Bush riding Ukraine as he would a horse, according to media reports here. Some of these items were discovered in storage at the National Exhibition Center in Kiev, leading the U.S. Embassy to protest to the government. Tigipko said the Yanukovych campaign had nothing to do with the material.

Opposition supporters also employ aggressive campaign tactics. Last week, hundreds forced their way into a hall where the Central Election Committee was considering a proposal that could favor Yanukovych. Alleging official meddling in the committee, the opposition partisans smashed windows and threw smoke bombs, according to accounts from the meeting. One allegedly waved a pistol. The commission backed down, rejecting plans to open hundreds of new polling stations in Russia.

Other serious violence has marred the campaign. A bomb exploded in a market in Kiev, the capital, killing one person and wounding 13. The government said it was planted by activists tied to the Yushchenko campaign; the opposition called the accusation a lie. The offices and cars of opposition figures have been bombed, with each side blaming the other.

"This atmosphere is so aggressive and so tense that I fear a single match could start a raging fire in the streets," said Vladimir Polokhalo, editor of Political Thought, a political science journal. "This is the dirtiest campaign in the history of Ukraine. And Ukraine has never had clean elections."

Opposition presidential candidate Viktor Yushchenko campaigning in Kirovograd, away from the traveling zoo that blocked planned rally spot.