KIEV, Ukraine - Ukraine's top security service denied today that it had any involvement with the dioxin poisoning of opposition leader Viktor Yushchenko, the leading candidate in Sunday's rerun of the presidential election.

In a statement posted on its Web site, Ukraine's State Security Service, or SBU, said that "it has no relation with the worsening" of Yushchenko's health.

Last week in an interview with The Associated Press, Yushchenko said he was probably poisoned at a Sept. 5 dinner with the head of the Ukrainian security service Ihor Smeshko and his deputy, Volodymyr Satsyuk, who later denied any involvement in the poisoning.

"The Ukrainian Security Service did not obtain a single official document that could provide ... a basis for the establishment of the time or the place or the fact of the candidate's poisoning," the statement said.

The poisoning dramatically disfigured Yushchenko's face but doctors have said he has recovered enough to campaign.

Ukrainian prosecutors and a special parliamentary committee are investigating and the state security service has expressed willingness to participate in the probe.

On Wednesday, Yushchenko told thousands of his orange-clad supporters massed at Kiev's Independence square for a giant rally that his victory is near.

"The doors have been opened. The only thing left for us is to step over the threshold," Yushchenko said during the open-air speech called to mark one month since the "orange revolution" protests.

He warned of a plot to disrupt the revote, but did not say who was behind it.

"The vote on Dec. 26 will not be an easy political walk," he said. "There are some forces preparing to disrupt and they are preparing brigades, groups who are readying to come to Kiev."

"We will come on this square, this stage, after the vote on Dec. 26 and will stay until our victory is celebrated," he said.

Fears of violence have been high ahead of Sunday's runoff, with rumors swirling that armed pro-Yanukovych supporters are poised to head to Kiev after the vote. Yanukovych's campaign staff has repeatedly denied the allegations.