"I plead not guilty," Germain Katanga replied to each of the 10 charges of war crimes and crimes against humanity against him, a plea echoed count-for-count by his co-accused Mathieu Ngudjolo Chui.
Katanga, 31, and Ngudjolo, 39, stand accused over an attack by their forces on the village of Bogoro in Democratic Republic of Congo's northeastern Ituri region that killed 200 people in February 2003.
They face charges of murder, rape, sexual slavery, using child soldiers, attacking civilians, pillaging and destruction of property.
Their trial, only the ICC's second and its first for murder, opened in The Hague at 0830 GMT.
The prosecution says more than 1,000 fighters of Katanga's Patriotic Resistance Force in Ituri (FRPI) and Ngudjolo's National Integrationist Front (FNI) entered Bogoro on February 24 six years ago "with one communicated and agreed goal: to erase the village".
They are alleged to have killed civilians, burning some alive in their homes, raped women, taken captive children to swell their ranks of child soldiers, and captured women as sex slaves.
"It is alleged that the roads to and from the village were blocked by the attackers in order to kill all civilians attempting to flee," states a court document.
Children were used in the Bogoro attack -- killing, and pillaging, says the prosecution.
Until the attack, the village had been controlled by rival Thomas Lubanga's Union of Congolese Patriots (UPC), blocking FRPI and FNI fighters and camps from the road to the key city of Bunia.
Katanga and Ngudjolo are both of Lendu ethnicity, while the Bogoro inhabitants were mostly Hema. Lubanga's own war crimes trial, the ICC's first, started in January.
Non-governmental bodies claim that inter-ethnic and militia violence in Ituri is about control of the area's gold mines, and has claimed 60,000 lives since 1999.
Katanga was surrendered by the DR Congo government to the ICC in October 2007, while Ngudjolo was arrested and transferred to The Hague in February 2008.
"I think I have already said: ever since I arrived here I have pleaded not guilty and I continue to plead not guilty," Katanga told presiding judge Bruno Cotte on Tuesday.
The ICC started operating as the world's first permanent and independent war crimes tribunal in 2002.
It has given victim status to 345 people -- including 10 former child soldiers -- who will participate in the trial through two lawyers.
The trial is expected to last several months, with the prosecution set to call 26 witnesses, 19 of whom will benefit from protective measures for their security.
ICC chief prosecutor Luis Moreno-Ocampo was set to make his opening statement later on Tuesday, followed by those of the victims and then of the defence.
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