Katanga, 38, was sentenced to 12 years in jail by The Hague-based International Criminal Court three years ago.
Katanga, 38, was sentenced to 12 years in jail by The Hague-based International Criminal Court three years ago for a 2003 attack on a village in the mineral-rich Ituri province, which left 200 people dead.
He finished serving a reduced sentence in January 2016 in the Democratic Republic of the Congo.
But he remained behind bars as Kinshasa readied to try him for "other crimes" committed in Ituri, near the Ugandan border.
Katanga appeared in court Friday along with six co-accused, wanted for "war crimes, crimes against humanity and participation in an insurrectional movement" in Ituri, where some 60,000 people died in fighting between 1999 and 2007.
The trial, which began in February last year, was interrupted soon after his lawyers argued that Katanga's prosecution could not proceed under the ICC's founding Rome Statute.
The statute says a sentenced person cannot be prosecuted in a country where he is serving his sentence without the ICC's approval.
The ICC gave the green light in April of that year, however the trial was postponed again two months later to allow Katanga's team to shore up its defence.
At the time, one of the accused had also testified that "there was nothing insurrectional" about their rebel group and that instead it "was working with the government to defend national territorial integrity" against militias supported by neighbouring countries.
For the first time since the trial began, families of the victims have been allowed to take part in the court proceedings.
The next public hearing has been scheduled for eight days.
A former member of the armed fighters of the Patriotic Resistance Forces in Ituri (FRPI), Katanga has offered his apologies to the victims, insisting he had turned his back on the militias which still wreak havoc in parts of the DR Congo.
Arrested in 2005 and then transferred to The Hague in 2007, Katanga was convicted of supplying weapons to his militia in the attack on the village of Bogoro where 200 people were shot or hacked to death with machetes.
He was acquitted of enforcing sexual slavery and using child soldiers.
Several senior officers in President Joseph Kabila's government, some of them former rebels integrated into the army, have been convicted of war crimes by Congolese courts.