The case is the first for the ICC at The Hague in the Netherlands, and the first international criminal prosecution to focus solely on child soldiers, according to human rights groups.
Lubanga showed no emotion as his lawyer said he pleaded not guilty to sending children under age 15 to fight in the armed wing of his Union of Congolese Patriots political party in 2002-03, reported the Associated Press.
Lubanga, 48, says he was trying to keep foreign fighters from taking Congo's vast mineral reserves from the eastern Ituri region.
Prosecutor Luis Moreno-Ocampo said he would seek a sentence "close to the maximum" of 30 years, according to the BBC.
Lubanga's trial is a defining moment both in the history of Congo's internal armed conflict and in the ICC's history, laying legal precedents for future trials. The ICC officially opened in 2002 as the first permanent war crimes court.
Lubanga was arrested in early 2006, and his trial was initially set for June 23, 2008. But a dispute over the use of classified information delayed the proceedings.
The United Nations gave the prosecution information that could have helped prove Lubanga's innocence, but the prosecutor did not share the information with the defense, believing it was classified.
ICC judges then ruled that the prosecution was obligated to share any evidence that may benefit Lubanga. A new trial date was set for Jan. 26.
The court's delays and the cumbersome evidence-gathering process raised questions about the ICC's efficiency and effectiveness, though analysts say there are no shortcuts in navigating international criminal trials like Lubanga's.
"There have been delays in the Lubanga trial that have come about as a result of the court leaning over backwards to protect Lubanga's rights," said David Scheffer, director of the Center for International Human Rights at the Northwestern University School of Law. According to Scheffer, prosecutors in war crimes tribunals often receive classified information from governments or organizations called "lead evidence" that is inadmissible in court but that prosecutors can use to gather other evidence for trial, pointing them in the right direction.
Scheffer said he believes speed bumps such as delays over the U.N. information are natural in a young institution like the ICC. "The delay is fodder to a lot of critics who think the justice is too slow and wonder why the trial hasn't been jump-started," Scheffer said. "But often these first trials trigger a lot of new issues ... and the defense throws a lot of things at the judges to see if they stick, because there's no precedent for it," he said.
Richard Dicker, director for International Justice at Human Rights Watch, said other hurdles come into play in war crimes trials. "Investigating widespread, systematic crimes in the context of ongoing armed conflicts -- many of which unfold in very remote regions with little infrastructure and transport -- is very difficult," he said.
Witness intimidation and prolonged instability also appeared to be obstacles to the Lubanga investigation because the militia leader still has strong pockets of support in the Ituri region that could have led to the spread of misinformation about the case, said Dicker.
Lubanga's supporters capitalized on the pre-trial complications last year, when the court considered halting the trial and releasing Lubanga, to spread rumors intended to frighten his former victims and their families, he said.
Dicker said the ICC's success will depend ultimately on how the information released during the trial is conveyed to the communities affected by the alleged crimes.
ICC public information and outreach coordinator Paul Madidi is organizing the court's efforts to ensure people in Ituri can follow Lubanga's case, but he said he has run into some obstacles, including the underlying ethnic tensions shading people's perceptions of the case in the region.
"There are a lot of people who still support Thomas Lubanga," Madidi said. "And we have some difficulty in keeping up dialogue with these people."
Madidi and his team have tried to improve their communication network in Ituri's communities, including arranging for a large screen TV to be put up in a main gathering area in Bunia, Ituri's capital, so people can watch the trial live. Several Congolese journalists will attend the trial and run radio reports through the national broadcaster.
"We'll also organize some meetings with people who have questions about the trial," Madidi said. "We have to continue to talk and bring views and good information to the people."