The International Criminal Court (ICC) has found the Congolese warlord Thomas Lubanga guilty of recruiting and using child soldiers between 2002 and 2003.
It is the court's first verdict since it was set up 10 years ago. He will be sentenced at a later hearing.
He headed a rebel group during an inter-ethnic conflict in a gold-rich region of Democratic Republic of Congo.
The prosecution accused him of using children as young as nine as bodyguards and fighters.
In a unanimous decision, the three judges said evidence proved that as head of the Union of Congolese Patriots (UPC) and its armed wing, Lubanga bore responsibility for the recruitment of child soldiers under the age of 15 who had participated actively on the frontline.
The BBC's Kasim Kayira at the hearing said Lubanga, who was arrested in 2005, was expressionless as the verdict was read out.
He has the right to appeal against his conviction on the three charges of recruiting children, enlisting children into rebel militia and using children in combat.
He faces between 25 and 30 years in jail as the prosecution asked for "close to the maximum" sentence.
US-based Human Rights Watch says more than 60,000 people were killed in the conflict between Hema and Lendu ethnic groups in Ituri, in north-eastern DR Congo.
It took the presiding judge Adrian Fulford 30 minutes to deliver the verdict.
He said a common plan was agreed by Lubanga and his co-perpetrators to create an army aiming to establish and maintain political and military control over Ituri.
"This resulted in boys and girls under the age of 15 being conscripted and enlisted, and used to participate actively in hostilities," the ICC said in a statement.
"The evidence demonstrated that children endured harsh training regiments and were subjected to hard punishment," AFP news agency quotes Judge Fulford as saying.
But the judge was also very critical of the prosecution's initial investigation, saying three intermediaries it had used in Ituri had led to unreliable evidence.
This had cost the court time and expense, he said.
The BBC's Anna Holligan in The Hague says the guilty verdict means victims and their families are entitled to apply for reparations from the court, which could be financial, symbolic or involve rehabilitation.
A representative of the non-governmental organisation Witness, which has worked with child soldiers in Ituri, said the verdict was "very significant both for those children who were recruited but also for the communities that were affected by the atrocities."
Bukeni Waruzi told the BBC it was likely to stop the militias' practice of recruiting children, which had continued even since Lubanga's arrest.
'Coming of age'
Three other men are accused of war crimes committed during ethnic strife in Ituri district 10 years ago, including General Bosco Ntaganda, who is now a general in the Congolese army.
The Congolese government says the verdict "marks a step forward for international justice".
But DR Congo's Justice Minister Luzolo Bambi said that its co-operation with the ICC took "into account the interests of justice [and] the interests of security".
The BBC's Kinshasa reporter Thomas Hubert says this means the DR Congo is unlikely to automatically hand over all suspects indicted by the court.
"It is high time the prosecutor go up the chain of command - who backed these armed groups in Ituri and in the Kivus, who provided them with weapons, who helped to train them?" Anneka Van Woudenberg from Human Rights Watch told the BBC's Focus on Africa programme.
"If those investigations are properly done that will of course take us to Uganda, Rwanda and indeed also to Kinshasa," she said.
UN Human Rights chief Navi Pillay welcomed the conviction as the "coming of age of the ICC".
Amnesty International said it showed the court could bring the world's worst offenders to justice.
"It will help to strip away the impunity they have enjoyed for crimes under international law because national authorities have consistently failed to investigate these crimes. This guilty verdict demonstrates that the ICC can step in to bring them to justice," Michael Bochenek, from the UK-based group said in a statement.
The ICC issued its first arrest warrant in 2005 against the Ugandan rebel leader, Joseph Kony, who remains at large.
His Lord's Resistance Army is accused of committing atrocities in several central African countries.
Others ICC suspects include Saif al-Islam Gaddafi, the son of the former Libyan leader, and several members of the Sudanese government, including President Omar al-Bashir.