SEOUL, Jan. 3 (Xinhua) -- Hundreds of South Koreans gathered outside the Japanese embassy in Seoul Wednesday to stage the protest against Japan's wartime sex slavery after a secret deal between the two countries was revealed last week.

About 400 South Korean citizens staged the so-called "Wednesday protest rally", which has continued for nearly 26 years, to demand the Japanese government's sincere apology and legal responsibility for Japan's wartime crime against humanity.

Around noon, some people sat on the cold mats and others were standing outside the Japanese embassy here in the cold freezing weather. People carried placards that read "Annul the 2015 Comfort Women Agreement" and "We Want Sincere Apology."

People laid yellow flowers at a girl statue, which symbolizes young women and girls who were kidnapped, forced or duped into sex servitude for Japanese military brothels during World War.

Historians say up to 200,000 young women, mostly from the Korean Peninsula, served as sex slaves for the Japanese army during the Pacific War.

The Wednesday demonstration was the first since the South Korean government under President Moon Jae-in, who took office in May 2017, revealed its review result last week over the agreement which was reached on Dec. 28, 2015 between the Park Geun-hye government and the Japanese cabinet led by Prime Minister Shinzo Abe.

The review result showed secret deals in the 2015 agreement, which involved Seoul's acceptance of Tokyo's demand for persuading the South Korean victims and their supporters to accept the agreement, not using the word of "sex slave" officially and helping remove the girl statue symbolizing the victims.

The secret deals were not made known to the general public as well as the victims at the time. No communications were made between then South Korean government and the "comfort women" victims before reaching the agreement in 2015.

"Comfort women" euphemistically refers to the victims of the wartime sex enslavement.

The then South Korean government under the impeached Park reached a "final and irreversible" agreement with Japan in return for receiving 1 billion yen (about 9 million U.S. dollars) dedicated to a foundation supporting the victims of South Korea.

President Moon said the comfort women issue cannot be resolved by the 2015 deal despite his burden that it was an official inter-governmental promise approved by the leaders of the two countries.

An advocacy group, which has supported the comfort women victims for almost three decades, claimed the return of the 1 billion-yen fund back to Japan and the annulment of the 2015 agreement.