They are accused of targeting very young girls, reportedly associating them with supernatural protection.
Those detained include the group's alleged leader, Frederic Batumike, who is a provincial deputy.
Most of DR Congo has recovered from nearly a decade of war but rebel violence continues in eastern areas.
Justice Minister Alexis Thambwe Mwamba said Mr Batumike and 74 of his fighters had been arrested last week for repeated rapes of about 30 very young girls near the village of Kavumu.
"The militia that works for him recruited a fetishist who advises the militiamen to rape very young girls to be assured of having supernatural protection," Mr Thambwe said.
The justice minister also said Mr Batumike was accused of ordering the killing of a German engineer, Walter Muller, and a Congolese human rights activist named as Evariste Kasali.
Q&A: DR Congo conflict
The director of the UN human rights office in the DRC, Jose Maria Aranaz, applauded the arrests.
"We welcome the authorities' action to put an end to these horrible practices and bring those responsible to justice," he said.
Some five million lives were lost in DR Congo's conflict between 1994 and 2003.(BBC)