During the five-day hearing, 78 former comfort women will give evidence of their suffering at the hands of the Japanese army during the 1930s and 1940s.
The tribunal is based on an indictment against Japan's wartime government, former Emperor Hirohito and senior military officers, for setting up and operating military brothels as part of the war effort. It follows years of fighting for redress and failed legal suits.
Representatives from nine Asian countries are in Tokyo for the mock trial which, while not legally binding, is highly symbolic.
Previous trials failed
The hearings come just days after Japanese courts rejected appeals by Korean and Filipino comfort women who were demanding an apology and compensation.
More than 450 participants, including victims, lawyers, judges and scholars from around the world are attending the tribunal.
It is being presided over by an international panel of five judges, led by Gabrielle Kirk-McDonald, an American who served as a judge at the former Yugoslavia war crimes tribunal.
A ruling will be handed down on Tuesday.
Women abducted
An estimated 200,000 to 300,000 women across Asia, predominantly Korean and Chinese, are believed to have been forced to work as sex slaves in Japanese military brothels.
Many were abducted, and some were barely in their teens. "I was taken at the age of 11," one former sex slave Kim Young-suk said.
One woman, Park Ok-ryun, was told she would work in a laundry. Instead, she was ordered to have sex with as many as 30 soldiers a day.
Although many women are now in the 70s - while others have died - they are still determined to fight for an official Japanese apology and compensation for their suffering.
The organisers, who include Asian women's groups from eight countries, hope to garner international support to seek redress.
Government denial
Although it has acknowledged the existence of comfort women brothels, the Japanese Government has consistently denied legal responsibility and refused to pay compensation.
For some though, even this acknowledgement goes too far. Right-wing groups have been protesting outside the tribunal, denying that such brothels existed.