Posters showing portraits of impeached President Park Geun-hye her aides and business men close to her are displayed during a vigil in Seoul (AP/Ahn Young-joon)

A Buddhist monk is critically ill in hospital after setting himself on fire to protest against South Korea's settlement with Japan on compensation for wartime sex slaves.

A hospital official says the 64-year-old monk suffered third-degree burns across his body and serious damage to his organs. He is unconscious and unable to breathe on his own.

Police say the man set himself ablaze late on Saturday during a rally in Seoul calling for the ousting of impeached President Park Geun-hye.

In his notebook, the man called Ms 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 Second World War military.

Many South Koreans say Japan's compensation is not enough.

AP