South Korean police arrested Liu Quang, 38, in January after he threw gasoline bombs at the Japanese embassy in Seoul. He is alleged to have set fire to Tokyo's Yasukuni Shrine in December, before arriving in Korea.
He said both attacks were in protest of Japan's refusal to apologize for forcing Korean women, including his maternal grandmother, to be sex slaves during World War II.
Upon his release Tuesday following his 10-month sentence for the Seoul attack, Liu was re-arrested and held for extradition to Japan to be tried for the shrine fire.
Hong Lei, a spokesman for China's Foreign Ministry, said in Beijing Tuesday Liu is a political prisoner and should be deported to China because his actions were a response to Japan's wartime crimes, The Chosun Ilbo reported Wednesday.
China says Liu's grandfather died resisting Japanese occupation in 1945.
Korean Foreign Ministry spokesman Cho Tae-young said Liu's pending extradition was based on a treaty with Japan and legal principles have priority.