Ukrainian officials say they are reopening a criminal investigation into opposition leader Viktor Yushchenko's mystery illness.
Changed man - Mr Yushchenko's appearance was transformed
On Saturday doctors in Vienna firmly linked his condition to poisoning.
They said extensive tests showed a form of dioxin had been used, leaving Mr Yushchenko's face disfigured.
At a news conference before leaving the clinic on Sunday, Mr Yushchenko thanked his doctors and said he was lucky to be alive after what he had been through.
"They've spent many days and nights with me and I am very
happy to be alive in this world today," he said.
Mr Yushchenko, 50, was taken ill in September as he campaigned for disputed elections that have now been declared invalid because of irregularities.
In October, prosecutors concluded that Mr Yushchenko was suffering the effects of a virus and that there was no evidence to suggest what he termed biological weapons were involved.
But media reports on Saturday said they were cancelling a previous decision to close the case.
Mr Yushchenko's doctors have 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, the doctors 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 the BBC's Helen Fawkes in Kiev says the doctors' announcement is likely to boost support for Mr Yushchenko at home.
The alleged poisoning is set to remain one of the key issues of the election campaign, she says. The second round is now being re-run on 26 December.
The most recent opinion poll has put the opposition leader 10 percentage points ahead of his rival, Viktor Yanukovych.
His supporters staged mass demonstrations against election fraud after Mr Yanukovych was initially declared the winner in November.
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.
Mr Yushchenko has accused the authorities of trying to poison him.
Yuliya Tymoshenko, a close associate of Mr Yushchenko, said those responsible could not escape the law if he is elected.
But the campaign manager of Mr Yanukovych, who is currently prime minister, rejected suggestions that he was involved.
Michael Zimpfer, the head doctor of the Rudolfinerhaus clinic where Mr Yushchenko underwent treatment, said on Saturday there was no doubt the disease had been caused by dioxins, mostly likely administered in something like soup.
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.