A Hutu rebel chief linked to atrocities in Rwanda and the Democratic Republic of Congo began fighting deportation from Germany where he was detained last week.
Ignace Murwanashyaka filed objections to being deported after his asylum application was rejected.
He has lived and studied in Germany for several years but his residence permit was withdrawn under UN Security Council sanctions which came into effect in October.
He was taken into custody on Saturday pending deportation.
Judges in the western city of Mannheim are expected to begin hearing his complaint on Tuesday but a decision may not be taken for two weeks, a court spokesman said.
In a separate move, Germany's federal prosecutor Kay Nehm is considering opening an investigation into Murwanashyaka for violating international law, a spokeswoman for his office said.
The United Nations mission in the Democratic Republic of Congo, known as MONUC, on Monday welcomed the arrest of Murwanashyaka, who is the leader of the Democratic Forces for the Liberation of Rwanda (FDLR), the political arm of the Rwandan Armed Forces (FAR) and the extremist Hutu militia Interahamwe.
MONUC said his forces were involved in the 1994 genocide of minority Tutsis and moderate Hutus in Rwanda in which 800,000 people died in the space of 100 days. They also carried out violence against civilians in northeastern DRC.
Murwanashyaka however is not directly implicated in the atrocities.
"It is now for the German authorities to decide the fate reserved for Ignace Murwanashyaka," MONUC said in a statement.
Rwandan Foreign Minister Charles Murigande called for Murwanashyaka to be prosecuted, possibly by the International Criminal Court (ICC) in The Hague, if German courts were unable or unwilling to charge him.
Failing that, he said Rwanda might seek to try Murwanashyaka in its own courts.