Judges have convicted a Congolese warlord of snatching children from the street and turning them into killers.

The ruling is the International Criminal Court's first judgment 10 years after it was established in The Hague as the world's first permanent war crimes tribunal.

Thomas Lubanga did not react as presiding Judge Adrian Fulford read out the verdicts Wednesday in The Hague. He now faces a maximum sentence of life imprisonment and will be sentenced following a hearing that will be scheduled later this year.

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Congolese warlord Thomas Lubanga is seen Wednesday behind his lawyers in the International Criminal Court, which found him guilty of war crimes. (Evert-Jan Daniels/Reuters)

Human rights advocates said the guilty verdicts against Lubanga should stand as a clear deterrent to armies around the world not to conscript children.

"This is a pivotal victory for the protection of children in conflict," said Anthony Lake, executive director of UNICEF.

"The conviction of Thomas Lubanga by the International Criminal Court sends a clear message to all armed groups that enslave and brutalize children: Impunity will not be tolerated."

Actress and activist Angelina Jolie watched the verdicts from the court's public gallery and called them a victory for former child soldiers.

"This is their day, where these children will feel there is no impunity for what happened to them, for what they suffered," Jolie said.

The trial, which began in January 2009, is the first at an international court to focus exclusively on the use of child soldiers. 

Lubanga, who claimed he was an innocent patriot, was accused of taking children from their families and using them as soldiers and bodyguards in 2002-03 in the armed wing of his political movement, the Union of Congolese Patriots, in a conflict in the Ituri region of Congo.

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A bodyguard stands behind the leader of the rebel Union of Congolese Patriots, Thomas Lubanga, in this June 3, 2003, photo taken during a rally by the rebel group. (Karel Prinsloo/Associated Press)

The judgment came at a time when the court is under scrutiny for its inability to arrest key war crimes suspects and its impotence in not being able to intervene in the bloody conflict in Syria.

The court was catapulted into the limelight last week by the viral video Kony 2012, which highlighted how it still has not had Ugandan rebel Joseph Kony arrested nearly seven years after indicting him for crimes including using child soldiers, murder and torture.

The court has no police force of its own and has to rely on states to enforce its arrest warrants.

It also can only open investigations in the 120 countries that have recognized its jurisdiction or at the request of the UN Security Council. Nations including the United States, China, Russia and Syria are not members.

That means it can't launch a probe into widespread allegations that forces loyal to Syrian President Bashar Assad are systematically committing atrocities to put down an anti-government revolt.

So far, all seven of the investigations launched by the court are in Africa.

The highest profile suspects among five in custody are former Ivory Coast president Laurent Gbagbo and ex-Congo vice-president Jean-Pierre Bemba. Sudanese President Omar al-Bashir has been indicted for genocide in Darfur but refuses to surrender to the court.

It took six years from the time Congo handed over Lubanga to his convictions, but ultimately the three-judge panel was unanimous in finding him guilty.

"The prosecution has proved beyond reasonable doubt that Mr. Thomas Lubanga is guilty of the crimes of conscripting and enlisting children under the age of 15 years and using them to participate actively in hostilities," said Fulford.

Lubanga, wearing an ivory-coloured robe and skull cap, sat with his hands clasped in front of him listening to the verdict and showed no emotion as Fulford declared him guilty.

As he left court flanked by guards, Lubanga nodded and smiled to supporters in the public gallery.

The victory for prosecutors came after the case twice nearly collapsed because of their failure to disclose evidence to defence lawyers and despite harsh criticism from judges in their written judgment.

Fulford said three intermediaries used by prosecutors to approach witnesses in Congo "persuaded, encouraged or assisted witnesses to give false evidence" and scrapped the evidence of three witnesses.