BUENOS AIRES — The disappearance of an indigenous rights activist in Argentina spurred violent clashes between demonstrators and the police on Friday and has raised dark memories of forced disappearances during the country’s dictatorship.

Relatives and supporters say the activist, Santiago Maldonado, was last seen during a confrontation on Aug. 1 between border police officers and supporters of a Mapuche indigenous community that was being displaced from an area in the Patagonia region that belongs to Benetton, the clothing company.

Family members said they feared that the security forces might have taken Mr. Maldonado into custody without disclosing his fate or whereabouts. The authorities say there is no evidence of foul play.

But the possibility that the government has secretly taken Mr. Maldonado, 28, into custody has transfixed and angered many Argentines, for whom the memory of mass disappearances and killings during the 1976-83 dictatorship remains fresh.