PARIS  More than six years after opening its doors, the International Criminal Court in The Hague began its first trial on Monday, as Thomas Lubanga, a former Congolese warlord, took his seat in the dock facing a crowded court and public gallery.

Supporters of the court have hailed the long-awaited trial as a momentous step for the tribunal, created to try large-scale human rights violations, while critics contend it has been far too long in coming. Mr. Lubanga was taken to The Hague almost three years ago, and his case has moved in fits and starts. Now, both sides see the trial as the institution’s first full-scale test, one that will be closely watched by lawyers and human rights activists the world over.

Mr. Lubanga, 48, once the leader of a powerful and violent militia, is accused of war crimes, including commandeering children under the age of 15 and sending them into war to maim and kill. He pleaded not guilty to the crimes on Monday, which prosecutors said occurred in 2002 and 2003 during ethnic fighting in the Ituri region of eastern Congo.

Turf wars within the court, bitter legal squabbles and irritation among the trial judges had almost torpedoed the case. Last July, as the trial was about to start, judges put a halt to the proceedings, citing legal and strategic errors by the prosecution, and said Mr. Lubanga should be set free, though he was ultimately kept in custody. The judges said the prosecution’s handling of evidence amounted to “wholesale and serious abuse” of the process and ruled that a fair trial was not possible at that point.