The former Georgian president Mikheil Saakashvili has led angry protests on Parliament after his supporters freed him from an attempted arrest by police in Kiev.
Mr Saakashvili served two terms in his native Georgia before moving to Ukraine in 2013, where he has reinvented himself as a prominent anti-corruption campaigner and opposition political figure.
His attempted detention at his home in Kiev has raised fears that Ukraine could be facing a fresh political crisis.
Mr Saakashvili had claimed on his roof to attract the attention of supporters after the SBU, Ukraine's Security Service, arrived in his home on Tuesday morning with a warrant to arrest him.
The SBU said he is wanted for "assisting members of criminal organizations or hiding their criminal activities." Prosecutors accuse him of being part of a Russia-linked plot to topple the country's President Petro Poroshenko.
Footage shows Mr Saakashvili being bundled out of the house and into a van, but a crowd of several hundred protesters then surrounded the vehicle to prevent it from driving.
Some scuffled with police or used cobblestones and construction rubble to build barricades. One protester climbed atop the van and waved the Ukrainian flag.
After 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 parliament building.
He also called on Ukrainians to rally on Maidan, Kiev's main square, the site of huge protests in 2013 and 2014, to demand Poroshenko's resignation.
Hundreds of his supporters joined the rally, with many shouting: "Kiev, rise up".
Mr Saakashvili is an antagonist to President Poroshenko, who appointed him as governor of Ukraine's Odessa region before the two had a falling-out.
Mr Saakashvili resigned in 2016, complaining that his efforts to root out corruption were being obstructed by officials. He has ignited dissent among some citizens by pointing to the slow pace of reforms pledged by the President.
In a growing standoff between the two men, Mr Poroshenko revoked Mr Saakashvili's Ukrainian citizenship while he was abroad in July - but Mr Saakashvili managed to re-enter the country across the border with Poland.
Although Mr Saakashvili has become a leading opposition figure, his protests and rallies typically only draw a few thousands people and he is not seen as a threat to Mr Poroshenko's Government.