Niger state governor Mohammed Umaru Bago said on Thursday that the miners, who were arrested earlier in the week on suspicion of illegal mining by the Nigeria Security and Civil Defence Corps, died in custody.
The civil defence corps said they died as a result of a suspected disease outbreak, but survivors told The Associated Press that the miners died of suffocation.
Dozens of young people, including miners and their relatives, gathered near the civil defence corps' office on Friday.
Police officers cordoned off the area and attempted to disperse protesters by firing tear gas canisters.
The protesters claim the miners were killed in retaliation for earlier attacks by some miners on the corps officers.
The Nigerian government suspended the commandant of Niger state's civil defence corps on Friday.
"It is an unfortunate incident, however, a full investigation will be conducted while the commandant under whose watch this happened remains suspended," the minister of the interior said in a statement.
Illegal mining is a serious problem in Nigeria, where analysts say it funds activities of armed groups, some of whom control the mines.
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