Slaves come from the poor communities in the north
A former slave is suing Niger's government, accusing it of failing to implement laws against slavery introduced in 2003.
Hadijatou Mani took the case to the Community Court of Justice of Ecowas, the Economic Community of West African States, based in Nigeria.
This is the first such case brought by a former slave in Niger.
The government says it has done all it can to eradicate the practice and says the problem has been exaggerated.
Ms Mani appeared in court with her lawyers, family members and anti-slavery activists.
The government was represented by a high-powered team including the prime minister, justice minister and speaker of parliament.
The case is due to last for five days.
Ms Mani says the government continues to legitimise the practice of slavery through customary law, which discriminates against women in direct conflict with the constitution and criminal code.
She was sold into slavery for around $500 when she was 12 years old.
She had to carry out domestic and agricultural work but also lived as a sex slave or "Sadaka" to her master who already had four wives and several other "Sadakas".
When she was released and decided to marry a man she had freely chosen, her master insisted she was in fact his wife.
A court later found her guilty of bigamy and sentenced her to six months imprisonment.
In 2003, Niger's parliament passed a bill making slavery a criminal offence with a possible sentence of up to 30 years imprisonment.
Local human rights group, Timidria claims there are still up to 43,000 people living as slaves.
However the government says these figures are exaggerated.
It says there is a caste system in Niger and members of lower castes have been mistaken for slaves.
Human rights groups say slave masters take children from their mothers when they are as young as two years old - to break the family bond as soon as the child is weaned.
The organisations believe this perpetuates slavery from one generation to the next.
The BBC's Idy Baraou in the capital, Niamey says many people in Niger hope the trial will open the way for more law suits, bringing this ancient practice to an end.