Coast Guard divers discovered the body in the Chubut river in southern Argentina, close to the site of an indigenous rights protest where Santiago Maldonado went missing on August 1.
Rights activists allege police beat the 28-year-old and took him into custody after he and a group of others blocked a road in Chubut, Patagonia, about 1,800 kilometers (1,100 miles) southwest of Buenos Aires. But authorities have never confirmed the arrest and deny any wrongdoing.
The demonstrators were demanding the release of a jailed Mapuche indigenous leader, as well as the return of ancestral lands that were sold to Italian clothing company Benetton but are claimed by the Mapuche.
Maldonado's disappearance sparked huge demonstrations in Argentina, with activists demanding the government of President Mauricio Macri find him alive. International rights groups also voiced concerns, while soccer star Diego Maradona and Oscar-winning composer Gustavo Santaolalla joined a social media campaign under the slogan: "Where is Santiago Maldonado?"
Some rights groups argue state security forces took Maldonado and that Macri's government covered up information about his whereabouts.
Maldonado's family has also blamed police for his disappearance. Their attorney, Victoria Heredia, suggested the body may have been planted, saying it was found in a place where it "was visible to the naked eye. We don't understand why it should appear in an area that had already been swept three times."
She said the body would be flown to Buenos Aires for formal identification and an autopsy.
"The family is asking for precautions to be taken" over how and where the body is stored until an autopsy is carried out, Heredia told reporters.
The case has revived distressing memories in Argentina, where security forces secretly detained, tortured and "disappeared" political opponents during the country's 1976-1983 dictatorship.
The discovery of the remains led political parties running in a midterm legislative election on Sunday to suspend their campaigns.
Activist groups said they planned to stage another march in front of the presidential palace in Buenos Aires on Wednesday to demand answers about Maldonado's fate.
nm/jm (Reuters, AFP, AP)