Former Ukrainian Prime Minister Yulia Tymoshenko was received on Saturday night by thousands of opposition supporters in central Kiev after being freed from detention, the BBC reports.
She has suffered from a back injury and addressed them from her wheelchair.
"You are heroes," she told the crowd on Independence Square, the focus of protests against President Viktor Yanukovych, and broke down in tears.
The speech came at the end of a dramatic day, with the president fleeing Kiev but refusing to quit.
Tymoshenko warned that the protesters should not think their job was done.
"Until you finish this job and until we travel all the way, nobody has the right to leave," she said, according to the BBC. "Because nobody could do it - not other countries, nobody - could do what you have done. We've eliminated this cancer, this tumor."
Tymoshenko was freed following a vote by parliament on Friday paving the way for her release.
She was sentenced to seven years in jail after a controversial verdict on her actions as prime minister.
Earlier Saturday, she left the hospital in the eastern city of Kharkiv, where she had been held under prison guard, and flew to Kiev.
She told journalists at Kiev airport that those behind violence "must be punished", the Interfax agency reported.
The Ukrainian health ministry says 88 people are now known to have been killed since February 18 in the latest series of clashes in Kiev.
On Friday, things seemed to have calmed down when Yanukovich and the opposition signed a deal to end the splintered country's worst crisis since independence.
Things took a dramatic turn on Saturday, when the parliament voted to remove Yanukovych and hold a presidential election on May 25.
Yanukovych said events in Kiev were a "coup" and vowed not to stand down.
He compared the actions of the opposition to the rise to power of the Nazis in 1930s Germany and claimed MPs from his party had been "beaten, pelted with stones and intimidated," according to the BBC.