The 64-year-old monk suffered third-degree burns across his body and serious damage to vital organs. He's unconscious and unable to breathe on his own, said an official from the Seoul National University Hospital, who didn't want to be named citing office rules.
The man set himself ablaze late Saturday during a large rally in Seoul calling for the ouster of impeached President Park Geun-hye, police said.
In his notebook, the man called Park a "traitor" over her government's 2015 agreement with Japan that sought to settle a long-standing row over South Korean women who were forced into sexual slavery by Japan's World War II military, police said.
Under the agreement, Japan pledged to fund a Seoul-based foundation that was set up to help support the victims. South Korea, in exchange, vowed to refrain from criticizing Japan over the issue and try to resolve the Japanese grievance over a bronze statue representing wartime sex slaves in front of its embassy in Seoul.
The agreement has so far come short of bringing a closure to the emotional issue. The deal continues to be criticized in South Korea because it was reached without approval from victims, and students have been holding sit-in protests next to the Seoul statue for more than a year over fears that the government might try to remove it.
On Friday, the Japanese government reacted angrily to the placing of a similar statue in front of its consulate in the city of Busan, announcing a recall of its ambassador to South Korea and suspension of economic talks.
At the time of the sex slave deal, Seoul said there were 46 surviving South Korean victims.