"When we ask ourselves today why this country has no president, the answer is that you were engaged in fraud," Yushchenko said. "What was the problem on 21 November? You stole 3 million votes."
Yushchenko, his face still scarred from dioxin poisoning, also accused Russia, which has backed Yanukovych, of interfering in Ukrainian politics.
Yanukovych, talking frequently in the Russian spoken in eastern areas seen as backing his candidacy, said Yushchenko has been supported by the United States and other Western countries.
He said a Yushchenko victory could divide the country, and called for whoever wins to form a national unity government.
"Viktor Andreyevich [Yushchenko], you think that you will win and become president of Ukraine. You are making a huge mistake," Yanukovych said. "You will be president of part of Ukraine. I don't want this. I want us both to unite Ukraine."
Yanukovych said his two years as prime minister have led to improved living standards for Ukrainians.
He also offered an apology for what he called "improprieties" in the election campaign.
The 100-minute debate was marked by several sharp exchanges and finger-pointing between the candidates, but the two shook hands cordially when it ended.