Hundreds of protesters chanting "Kiev, rise up" have blocked Ukrainian police as they tried to arrest former Georgian president Mikheil Saakashvili.
He later escaped with help from supporters and led them on a march towards parliament, where they planned to call for President Petro Poroshenko to resign.
The detention of Mr Saakashvili, now an anti-corruption crusader in his adopted home and arguably the country's most popular opposition politician, has raised fears that Ukraine could be facing its most acute political crisis since the 2014 revolution.
Ukrainian prosecutors accuse him of colluding with Ukrainian businessmen who have ties to Russian intelligence as part of an effort to topple the president.
Mr Saakashvili poses a threat to Mr Poroshenko, who appointed him as governor of Ukraine's Odessa region before the two had a falling out.
In 2016 Mr Saakashvili resigned, complaining that his efforts to root out corruption were being obstructed by officials.
When the SBU, Ukraine's Security Service, went to detain Mr Saakashvili at his home in Kiev on Tuesday, he climbed on to the roof and reportedly threatened to jump off.
SBU officers went after him, detained him and led him to a waiting van.
Several hundred supporters surrounded the van, refusing to let it drive off.
Footage from the scene showed protesters picking up cobblestones and construction rubble to build barricades.
One protester climbed on top of the van and waved the Ukrainian flag.
After Mr Saakashvili escaped, he told his supporters that he would "lay down his life for the freedom of Ukraine" and called on them to follow him to the Supreme Rada.
He also called on Ukrainians to rally at Kiev's main square, the epicentre of protests in 2013 and 2014, to demand Mr Poroshenko's resignation.
Footage showed Mr Saakashvili with the yellow-and-blue Ukrainian flag around his neck marching in central Kiev, surrounded by crowds.
He was Georgia's president for nearly a decade before he was prevented from running again by term limits. He left the country in 2013.
Mr Poroshenko revoked Mr Saakashvili's Ukrainian citizenship in July.
Mr Saakashvili forced his way across Ukraine's border with Poland earlier this year, with help from protesters.
His stand-off with Mr Poroshenko ignited long-simmering popular discontent with the slow pace of reforms the latter has promised.
The security service said in a statement that Mr Saakashvili is facing a criminal investigation for "assisting members of criminal organisations or hiding their criminal activities".
Ukrainian prosecutor general Yuri Lutsenko said on TV that prosecutors have evidence that Mr Saakashvili's representative received $500,000 from Ukrainian businessmen with ties to Russia to finance protests.
Mr Saakashvili spearheaded several protests in Kiev, but they typically drew fewer than 4,000 people.
At one of the rallies, Mr Saakashvili called on Mr Poroshenko to resign.
"All the rallies were financed by foreign oligarchs that aimed to seize power by illegal means," Mr Lutsenko said.
The prosecutors plan to ask the court to place Mr Saakashvili under house arrest, he said.
