Ukraine's rival presidential candidates scheduled overlapping campaign trips to the eastern industrial city of Kharkiv on Friday, sparking fears of more tension ahead of a Dec. 26 rerun election as opposition leader Viktor Yushchenko (search) warned of possible provocation.

Campaign managers for Prime Minister Viktor Yanukovych (search) said the candidate intended to meet his activists in a local theater and hold a rally in the city center.

Meanwhile, Yushchenko was scheduled to visit Kharkiv and another industrial center, Zaporhizhia, to meet local businessmen in an attempt to expand his support base from the Ukrainian-speaking west to eastern Russian-speaking areas. Yushchenko's campaign said no mass rallies were planned.

Local police said that they "have undertaken all precautions" to avoid trouble. "Their routes will not intersect in Kharkiv," duty officer Yuriy Morozykov said.

The two are vying for votes with just nine days to go until the Supreme Court-ordered revote on Dec. 26. The court's finding that a Nov. 21 runoff election was fraudulent came as hundreds of thousands of Yushchenko supporters staged street protests in Kiev.

Yushchenko warned Thursday that his opponent could try to spark trouble in the rerun, saying he knows about "provocations being prepared in the eastern regions," where Yanukovych enjoys strong support. He did not elaborate.

The tense election campaign has been roiled by revelations about Yushchenko's dioxin poisoning — an attack he described as an assassination attempt.

Yushchenko told The Associated Press in an interview that he was most probably poisoned at a Sept. 5 dinner with the head of the Ukrainian security service Ihor Smeshko and his deputy, Volodymyr Satsyuk.

The poisoning dramatically disfigured his face, and the most recent tests indicate the level of dioxin in his blood is more than 6,000 times higher than normal. Doctors have said he has recovered enough to campaign.

Meanwhile, Abraham Brouwer, professor of environmental toxicology at the Free University in Amsterdam, told The Associated Press that the dioxin was TCDD (search), chemically known as tetrachlorodibenzoparadioxin (search), the most harmful known dioxin.

TCDD is a key ingredient in Agent Orange (search), a herbicide that was blamed for myriad health problems in U.S. veterans of the Vietnam War.

Mykola Polyshchuk, a lawmaker and a member of the commission tasked with investigating the poisoning, said he could not say "from where the dioxin could have been procured."

"I don't know if there is a place in Ukraine that produces it. Maybe in Russia," Polyshchuk told the AP.

He said that "there are certain forms of dioxins that are produced in Russia."

In a separate interview with the AP on Thursday, Yanukovych said he felt sorry for the opposition politician, but added he does not "want to be associated with the part of the authorities Yushchenko was talking about."

Yanukovych has repeatedly tried to distance himself from Kuchma, claiming that the backing of the scandal-tainted president had harmed his candidacy.

Yanukovych's campaign manager Taras Chornovyl said Friday that he cannot exclude potential troubles after the Dec 26 vote.

"People are so indignant. Anything can happen," Chornovyl told The Associated Press.

Yanukovych "cannot take responsibility for the actions of 15 million supporters" if they "don't agree" with a potential Yushchenko election victory, he said.

Chornovyl dismissed the Supreme Court's ruling after the Nov. 21 runoff as "legal nihilism made under pressure" from Kuchma's administration.

On Thursday, Yushchenko said he was confident that Ukraine would not split, and said that regional officials in eastern provinces who had pushed for self-rule as part of their efforts to support Yanukovych should be punished.

In an interview with private Kiev-TV, Yushchenko later said that outgoing President Leonid Kuchma has not sought any guarantees of immunity from prosecution after he transfers power following the rerun. Yushchenko hinted that Kuchma could be prosecuted over the slaying of an investigative journalist four years ago.

Kuchma has denied any involvement in the death of Hrihoriy Gongadze, the journalist whose slaying and beheading prompted wide-scale protests against Kuchma's government.

The Central Election Commission announced Friday more than 6,000 international and local monitors have been registered for the vote.