Former Congolese militia leader Thomas Lubanga has pleaded not guilty to charges of using child soldiers, at the International Criminal Court (ICC).
He faces six charges of recruiting and using hundreds of children aged under 15 to fight in DR Congo's brutal five-year conflict, which ended in 2003.
Opening the prosecution, Luis Moreno-Ocampo said Mr Lubanga had used the children to "kill, pillage and rape".
The case is the first to come to trial before the ICC in The Hague.
Its opening follows a seven-month delay, as judges and prosecutors at the world's first permanent war crimes court disputed confidential evidence.
Prosecutors say child soldiers enlisted for Mr Lubanga's ethnic Hema militia were used to kill members of the rival Lendu ethnic group, or as his bodyguards.
Giving his opening statement to a three-judge panel, Mr Moreno-Ocampo said the prosecution would prove that between 1 September 2002 and 13 August 2003, Mr Lubanga "systematically" recruited children under 15 as soldiers.
He said: "Lubanga's militia recruited, trained and used hundreds of young children to kill, pillage and rape.
"The children still suffer the consequences of Lubanga's crimes. They cannot forget what they suffered, what they saw, what they did. They were nine, 11, 13 years old."
Mr Moreno-Ocampo said some of the children were now using drugs to survive and some had become prostitutes.
He showed the court video footage of Mr Lubanga at a training camp, apparently in the company of young men and children, some of them dressed in military uniform.
Children were abducted on their way to school or to sports fields, Mr Moreno-Ocampo said, and young girls were taken as "sexual slaves" by militia commanders as soon as they reached puberty.
Given the damage done to his victims, Mr Moreno-Ocampo said he would seek a "very severe" sentence for Mr Lubanga, of "close to the maximum" of 30 years.
The prosecution plans to call 34 witnesses - among them former child soldiers, ex-militia members and experts - in the course of the trial, which is expected to last several months.
The first witness, who is due to take the stand on Wednesday, will be a former child soldier. His testimony will be followed by that of his father.
The BBC's Geraldine Coughlan at The Hague says 19 of the witnesses will testify anonymously behind a screen with their voices distorted.
She says the case is the first in history to focus exclusively on the use of child soldiers as a war crime and the first time victims will have been allowed to participate fully in an international trial.
Mr Lubanga insists he was trying to bring peace to Ituri, a region in eastern Democratic Republic of the Congo wracked by years of conflict between rival groups seeking to control its vast mineral wealth.
He was the leader of the Union of Congolese Patriots (UPC) and its armed wing at the time of the alleged crimes in 2002-2003, and still has strong support among his Hema community in Ituri.
The proceedings of the ICC trial are being given wide media coverage across the Ituri region.
The BBC's Karen Allen in eastern DR Congo says some 400 people, including former combatants, have gathered to watch proceedings on a giant screen in the regional capital, Bunia.
She says children are still being forced to fight in the separate conflict in the neighbouring North Kivu province.
'Marijuana and witchcraft'
A total of 93 alleged victims are being represented by eight lawyers at Mr Lubunga's trial.
The prosecution says children were snatched as they walked to school and suffered beatings and other abuses.
Many were plied with marijuana and told they were protected by witchcraft, according to human rights groups.
More than 30,000 children were recruited during the fighting, which saw some 60,000 people lose their lives.
The ICC trial sends a clear signal to rebel leaders and army commanders around the world who have frequently been able to commit atrocities on the battlefield with impunity, says the BBC's Africa analyst Martin Plaut.
Separately, judges at the court are expected to decide soon whether to issue an arrest warrant for Sudanese President Omar al-Bashir, who is accused of genocide in Darfur.