People rushed to help the unidentified man who was attending a demonstration over Japanese soldiers' use of Korean women as sex slaves in the 1930s and 40s.
"At first, I thought it was the trees that had caught on fire," said Kim Do-yeon, a student who witnessed the incident. "But when I saw it closely, it was the older man who was covered in fire and was burning."
Video from the scene showed people trying to beat the flames out with blankets and then spraying the man with a fire extinguisher. Police and rescue workers later arrived on the scene.
The man is being treated for non-critical burns at Seoul National University Hospital, police said.
Continuing anger over comfort women issue
There are regular protests outside the embassy over Japan's treatment of Korean women before and during World War II. Many historians say that approximately 200,000 women from Korea, China and other parts of Asia were forced by Japan to become sex slaves.
The Japanese government has formally apologized on numerous occasions for the atrocities against the women, who are known as comfort women. Japan helped establish the Asian Women's Fund in 1995, which is supported by government funds and provides assistance to comfort women.
But Tokyo has resisted direct compensation to the victims, prompting activists and former comfort women to say Japanese leaders are avoiding officially acknowledging what happened. Only a few dozen of the women are still alive today.
Anger has also been fueled by the stance of Japan's conservative Prime Minister, Shinzo Abe, who has said he does not believe the women were coerced by the Japanese military to work in its brothels.
The protest Wednesday outside the embassy in Seoul was scheduled to be the biggest rally this year ahead of the 70th anniversary of the end of World War II.