He left a note urging authorities to arrest the scandal-hit president for committing "treason".
A South Korean Buddhist monk is in critical condition after setting himself on fire during a mass protest against the impeached President Park Geun-Hye, officials said Sunday.
The monk, in his 60s and whose name was not released, set himself alight late Saturday night in central Seoul where hundreds of thousands returned to the streets for the eleventh week to demand Park's ouster.
He left a note urging authorities to arrest the scandal-hit president for committing "treason", the Yonhap news agency reported.
Park was impeached by parliament last month over an influence-peddling scandal that sparked a storm of public fury and nationwide protests, and the Constitutional Court now has to decide whether to confirm the impeachment.
The monk also slammed Park as a "traitor" for forging a deal with Japan to settle compensation for women forced into sexual slavery for Japanese troops during the World War II, according to Yonhap.
Critics say the 2015 deal did not go far enough in holding Japan responsible for wartime abuses. Tensions between the two countries spiked on Friday when Tokyo recalled its ambassador over a statue of a "comfort woman".
The monk suffered third-degree burns across his body and remains unconscious, according to police and staff at Seoul National University Hospital where he is being treated.
Self-immolation is not unheard of as a means of protest in the South, and was particularly common during the pro-democracy movement of the 1980s and early 90s when a number of activists set themselves on fire during public demonstrations.
Park is accused of colluding with her secret confidante, Choi Soon-Sil, to coerce top local firms to "donate" tens of millions of dollars to non-profit foundations which Choi then used as her personal ATMs.
The president is also accused of letting Choi, the daughter of a shady religious figure who was also close to Park for decades, meddle in state affairs including nomination of top officials.
Both Park and Choi -- who is currently on trial -- have denied any wrongdoing.