Prosecutors at the international criminal court at The Hague are due to present their case on Monday against a child soldier turned militia leader from northern Uganda accused of committing war crimes including rape and murder.

Dominic Ongwen, once a feared commander of the Lord’s Resistance Army, appeared last month before the court to plead not guilty, saying that as a child soldier taken by force from his home by the organisation he was a victim of its atrocities, not a perpetrator.

The trial of is one of the most momentous in the ICC’s 14-year history, and raises difficult questions of responsibility and blame.

Ongwen, who is 41, was abducted by the LRA at the age of 10. He is the first former child soldier to face trial at the institution and the first defendant to be both alleged perpetrator and victim of the same crimes.

He told the court in December that he was “one of the people against whom the LRA committed atrocities” and should not be on trial.

“In the name of God, I deny all these charges… I am not the LRA … It is the LRA who abducted people in northern Uganda. It is the LRA who killed people,” Ongwen said, speaking through an interpreter.

But Fatou Bensouda, who will lead the prosecution team, told the court at the earlier hearing that past victimisation as a child might be a mitigating factor in sentencing but was not a defence of Ongwen’s alleged decision to “wholeheartedly” embrace violence.

“Ongwen was directly involved in many attacks on civilians … He knew that the crimes he committed were part of widespread and systematic attacks … He played a crucial role in the abduction of children in order to maintain the fighting strength of the LRA,” she told the court.

The trial may last many years, officials have warned. The prosecution alone has signalled it will call more than 70 witnesses.

The charges against Ongwen include murder, attempted murder, torture, rape, sexual slavery, the alleged conscription of children under the age of 15 into an armed group and, for the first time, “forced pregnancy” and “forced marriage”.

If convicted, Ongwen faces up to 30 years in prison, or a life sentence could be imposed.

Led by Joseph Kony, a former rebel leader who claimed to be religiously inspired, the LRA waged war across five countries in east and central Africa for nearly 30 years.

The group has been blamed for the deaths of about 100,000 people and the abduction of 60,000 children. It relied on the abduction of largely defenceless villagers and refugees, including children, to provide labour and combatants. Girls were forced into sexual and domestic slavery while boys were forced to take up arms.

Most of the charges against Ongwen focus on attacks on refugee camps between 2002 and 2005. One of the worst involved a four-day raid by the LRA on camps in north-eastern Congo in December 2009, in which about 350 civilians were killed and another 250, including at least 80 children, were abducted.

People watch a screening of the start of the trial of Dominic Ongwen in Lukodi, Uganda, in December. Photograph: Isaac Kasamani/AFP/Getty Images

Defence lawyers will argue that much of the evidence against Ongwen is unreliable and say their client was brutalised and traumatised after being abducted.

“He was tortured … forced to watch people being killed, used for fighting as a child soldier. Even the prosecution have said that what he went through is a serious mitigating factor,” Thomas Obhof, a US lawyer based in Uganda and part of the defence team, said in December.

Ledio Cakaj, the author of When the Walking Defeats You, which documents the experiences of a bodyguard to Kony, said the trial raised a very tricky issue as “a lot of these people of the LRA, who are considered as the bad guys, were in some sense victims in the first place”.

Of the five senior LRA leaders indicted by the ICC more than a decade ago, only Ongwen and Kony are still alive. Despite a $5m (£3.5m) reward for information leading to his capture, Kony remains elusive.

The case is important for the embattled ICC, which was founded in 2002 to bring to justice perpetrators of crimes that local criminal systems cannot deal with.

The institution has repeatedly faced criticism by some in Africa who see it as a racist or imperialist. Nine out of the 10 cases being investigated by the ICC involve alleged crimes in Africa, where several countries have announced their withdrawal from the court.

Defenders say the majority of the investigations have followed an explicit request or grant of jurisdiction from the government in the country where the crimes were committed, as in the case of Uganda.

The trial of Ongwen has been hailed by human rights campaigners as a “significant first on justice for LRA atrocities”.