He said protests arising after the disputed presidential vote were comparable to the fall of the Berlin Wall or the USSR.

On Saturday the doctors firmly linked his condition to poisoning. They said extensive tests showed a form of dioxin had been used, leaving Mr Yushchenko's face disfigured.

"They've spent many days and nights with me and I am very happy to be alive in this world today," he said at a news conference with the doctors as he left the Vienna clinic.

Earlier Ukrainian prosecutors said they were re-opening an investigation into Mr Yushchenko's illness.

In October, they had concluded that Mr Yushchenko was suffering the effects of a virus.

'Different country'

Mr Yushchenko's supporters staged mass demonstrations against fraud in November's presidential elections after his rival, Viktor Yanukovych, was initially declared the winner.

The elections were later declared invalid. The second round is now being re-run on 26 December.

On Sunday, Mr Yushchenko said the regime which had been in power for the past 14 years was now into its last days and the world had seen a "different country".

"I think it would be appropriate to compare this to the fall of the Soviet Union or the fall of the Berlin Wall," he said.

"Now every corner of the world knows Ukraine," he said.

The most recent opinion poll has put the opposition leader 10 percentage points ahead of Mr Yanukovych, who is currently prime minister.

Mr Yushchenko has accused the authorities of trying to poison him, but Mr Yanukovych's campaign manager rejected suggestions that he was involved.

'Deeply concerned'

Mr Yushchenko's doctors described the poisoning as serious and said that, if left untreated, it could have killed him.

His blood and tissue registered concentrations of dioxin 1,000 times above normal levels, they said.

There appeared to be little lasting damage to Mr Yushchenko's internal organs, though experts say it could take more than two years for his skin to return to normal.

But in Russia, a health ministry official questioned their conclusions.

"Dioxin is not a poison with an immediate effect," Yuri Ostapenko said in an interview with Moscow Echo radio.

"Toxicity builds up over years, dozens of years, and it is impossible to receive a dose one day that would poison you the next."

Russian President Vladimir Putin has been backing Mr Yanukovych in the Ukrainian election.