"I was leaving a city council meeting and saw a man burning. People were trying to extinguish the flames with fire extinguishers," eyewitness Paulina Piechna-Wieckiewicz told AFP.
"Leaflets were strewn around him on the ground and a speaker was broadcasting the song "I Love and I Understand Freedom," by a 1990s Polish rock band, she added.
Warsaw police spokeswoman Magdalena Bieniek told AFP that "the incident took place around 4.00pm (1400 GMT) when a man poured a liquidon himself and then set it on fire."
"He was taken to the hospital in a serious condition," she said, adding that "an investigation was launched to determine his identity and motivations."
"We found leaflets around him and we want to determine if they belonged to him," she added.
Piechna-Wieckiewicz published the two-page leaflet on Twitter.
"I love freedom first and that is why I decided to immolate myself and I hope that my death will shake the consciences of many people," wrote its unidentified author.
He accuses Jaroslaw Kaczynski, head of the governing Law and Justice (PiS) party, of "having blood on his hands".
The leaflet also accuses the PiS government of limiting civil rights, violating the constitution, and destroying the constitutional court and the independent judicial system.
A small group of people later held a vigil in front of Warsaw's landmark Palace of Culture, where the incident happened.
Poland's foreign minister in August denied accusations his EU country was drifting towards authoritarianism amid a string of government reforms that have triggered mass protests at home and warnings from the EU.
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