Ukrainian former Prime Minister Yulia Tymoshenko has urged opposition supporters in Kiev's Independence Square to continue their protests.
Ms Tymoshenko, who has a back injury, addressed crowds from a wheelchair after being freed from detention.
"Until you finish this job... nobody has the right to leave," she said.
Her speech came at the end of a dramatic day that saw President Viktor Yanukovych removed by MPs and fleeing Kiev, but refusing to stand down.
Ms Tymoshenko broke down in tears as she told cheering supporters late on Saturday: "You are heroes.
"Because nobody could... do what you have done," she said. "We've eliminated this cancer, this tumour."
But while she was hailed by many in the audience, she does not enjoy universal support among the opposition, says the BBC's David Stern in Kiev.
Before she went into prison in 2011, her popularity ratings were dropping and many Ukrainians blame her in part for the chaos of the post-Orange Revolution years, or see her as a member of Ukraine's corrupt elite.
Dozens of people walked away in disgust when she appeared on the stage, the BBC's Tim Wilcox in Independence Square reports.
A vote by parliament on Friday paved the way for her release.
She was sentenced to seven years in jail after a controversial verdict on her actions as prime minister.
Earlier on 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 reports.
The health ministry says 88 people are now known to have been killed since 18 February.
Opposition seize control
Ukraine's parliament voted on Saturday to remove President Yanukovych and hold a presidential election on 25 May.
The parliamentary vote came after police stopped guarding presidential buildings, allowing protesters into the presidential country residence outside Kiev, and parliament made new high-level appointments.
Mr 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".
The opposition is now in effective control of the capital Kiev, with Mr Yanukovych's last known whereabouts in Kharkiv after travelling there late on Friday night.
Media reports have quoted Ukrainian officials as saying Mr Yanukovych was stopped by border police while attempting to fly to Russia aboard a private plane.
Ms Tymoshenko's release has been a key demand of the protest movement.
The glamorous, fiery orator who helped lead the Orange Revolution - Ukraine's revolt against a controversial election in 2004 - was convicted of criminally exceeding her powers when she agreed a gas deal with Russia which was seen to have disadvantaged Ukraine.
She has always insisted the charges were untrue, inspired by Mr Yanukovych, the man she helped oust in 2004 who returned to defeat her in the 2010 presidential election.
Her release was one of the conditions of the EU-Ukraine trade pact that President Yanukovych rejected last year - triggering the protests that led to the current crisis.
European Commission President Jose Manuel Barroso welcomed Ms Tymoshenko's release, calling it "essential for a democratic Ukraine".
The US also hailed the move. "We continue to urge an end to violence by all sides and a focus on peaceful, democratic dialogue,'' White House spokesman Jay Carney said in a statement.
In a telephone conversation with Russian Foreign Minister Sergei Lavrov, US Secretary of State John Kerry noted the "peaceful atmosphere" prevailing in Kiev after the departure of President Yanukovych, officials said.
In April 2013 the European Court of Human Rights ruled that Ms Tymoshenko's pre-trial detention had been "arbitrary and unlawful", though the judges did not rule on the legality of her actual conviction for the 2009 gas deal.
They did not explicitly support her claim that her detention was politically motivated, nor did they accept her allegations of physical maltreatment and medical neglect in prison.
A pact signed on Friday by Mr Yanukovych and opposition leaders now seems to have been overtaken by events.
It called for the restoration of the 2004 constitution and the formation of a national unity government.
The agreement failed to end the protests huge crowds remaining in Independence Square on Saturday calling for Mr Yanukovych's resignation.