Thomas Lubanga, the leader of an armed group in the Democratic Republic of Congo, has pleaded not guilty to war crimes charges in the first case to be tried by the International Criminal Court in The Hague.
He is accused of leading an armed group that committed atrocities during a conflict in the African state that led to the deaths of 60,000 people.
Lubanga is also alleged to have recruited hundreds of child soldiers to fight in his Union of Congolese Patriots (UPC) in the civil war in the country's eastern Ituri region between September 2002 and August 2003.
Much of the fighting there was between the UPC, composed mainly of ethnic Hema, and people from the Lendu ethnicity, largely represented by a group called the Nationalist and Integrationist Front.
The conflict has centred around control of one of the most lucrative gold-mining territories in the world.
'Symbolic' moment
Humanitarian groups say the conflict has created hundreds of thousands of refugees.
"From what is known about his [Lubanga's] defence it is likely to present him as a politician ... and it will be up to the prosecution to disprove that," Johna Hull, Al Jazeera's correspondent, reported from outside the ICC building in The Hague.
The trial, the first since the court came into operation in July 2002, is being seen as a crucial test for the tribunal as it seeks to establish itself as a means of bringing war criminals to justice.
"More than just a trial, the ICC would have this moment be seen as a symbolic one for international law and international justice - a message to war-mongers everywhere that they can no longer commit genocide and crimes against humanity with impunity," Hull said.
The first witness in Lubanga's trial, a former child soldier, is expected to take the stand on Wednesday, followed by his father.
Lubanga 'destitute'
The prosecution has listed 34 witnesses, including former child soldiers, ex-members of groups involved in the Ituri fighting.
The prosecution also plans to call on an array of experts in such speciality areas as determining the age of a child from x-rays of bones.
Lubanga, who is being held at a UN detention centre in the seaside suburb of Scheveningen in The Hague, has been declared destitute by the court, which is paying for his defence team.
The International Criminal Court is the world's first permanent tribunal to prosecute war crimes, crimes against humanity and genocide.
The trial is expected to last between six and nine months.