Rwanda genocide suspect Ladislas Ntaganzwa has been flown to Rwanda from Democratic Republic of Congo for trial.
Arrested in eastern DR Congo in December, Mr Ntaganzwa, 53, is accused in a UN indictment of genocide, crimes against humanity and violating the Geneva Conventions.
He is alleged to have helped form a Hutu militia “to exterminate” Tutsis while mayor of the town of Nyakizu.
Some 800,000 people, mostly ethnic Tutsis, died in the 1994 genocide.
Militias from the majority Hutu ethnic group massacred Tutsis and moderate Hutus.
The US had issued a $5m (£3.2m) reward for Ntaganzwa’s arrest, calling him “one of the main instigators of the genocide”.
The indictment by the UN-backed International Criminal Tribunal for Rwanda (ICTR) describes his involvement in the killing of more than 20,000 Tutsis between 14 and 18 April 1994.
It says he “substantially participated in the planning, preparation and execution of the massacre”.
He told a group including Hutu civilians to surround Cyahinda parish, in southern Rwanda, “so that no Tutsis could escape and told them to kill Tutsis”, the indictment alleges.
Mr Ntaganzwa is also “alleged to have orchestrated the rape and sexual violence committed against many women”.
His lawyers have yet to comment.
Reacting to his transfer from neighbouring DR Congo, the head of Rwanda’s Genocide Fugitive Tracking Unit, Jean-Bosco Siboyintore, said: “We are very happy to see this effected.”
Eight suspects remain at large and are still wanted by the UN for their alleged role in the genocide.