Mr. Maldonado, an activist who has campaigned about the forcible eviction of members of the indigenous Mapuche community from land in Chubut province in southern Argentina, vanished on August 1.
Witnesses claim he was taken away by the military police but the government of President Mauricio Macri offered a 500,000 peso ($27,000) reward for information about his disappearance.
The Mothers of the Plaza de Mayo group, which campaigned for years for an explanation about what had become of the so-called Desaparecidos (Disappeared) after the 1977 coup, have lent their support to calls for him to be freed.
"We want to embrace Santiago again and make sure that all officials responsible for the disappearance go to jail," said Hebe de Bonafini, from the Mothers of the Plaza de Mayo.
Mr. Maldonado was taken from a Mapuche community at Cushamen which faces eviction.
The territory, which is the size of Luxembourg, is claimed by the Mapuche, who were driven out of large parts of Argentina in the 19th century in a campaign led by General Julio Roca. Benetton has tried to oust a group from the Mapuche Ancestral Resistence movement and their supporters, which has been dug in since 2015.
Mr. Maldonaldo had traveled from his home in the city of La Plata, near Buenos Aires, to support the Mapuche, who were demanding the release of their leader, Jones Huala, who faces extradition to Chile on charges of terrorism related to land disputes on that side of the border.
The former President of Argentina, Cristina Fernandez de Kirchner tweeted a photograph of Mr. Maldonado on Saturday (August 6).
Santiago debe aparecer. Y debe aparecer con vida. pic.twitter.com/pon2fdox1t— Cristina Kirchner (@CFKArgentina) 6 August 2017
Tweet: "Santiago should appear and appear alive"
Mrs. Kirchner was beaten by Mr. Macri in the 2015 general election and he has introduced an austerity campaign which has resulted in huge government cuts to services which has made him hugely unpopular.
Mr. Macri, a right-winger, has also been criticized for being overly friendly with the United States and with foreign businesses who want to exploit Argentina.
In October 2017 he faces crucial mid-term elections for the Argentine congress and on Sunday (August 13) there will be primaries ahead of that poll.
Karen Granero, the editor of Sustenta magazine in Buenos Aires, said the Maldonado case had put Argentina into "full crisis" mode.
"There will be a big demonstration on Friday to demand Santiago Maldonado be found and found alive," she told Sputnik.
"The government says it's not true that he was taken by the military police but there are elections this Sunday and the government is facing their worst opinion poll numbers," said Ms. Granero. Industry has collapsed, employment has fallen, inflation has gone overboard with the government claiming 22 percent annually but other studies saying it is more like 40 percent. Ordinary people are not able to pay their bills," she told Sputnik.
"This demonstration might be the nail in the coffin," Ms. Granero told Sputnik.
Argentina has been a democracy since 1983, when the military handed over power. But many victims of the disappearances between 1977 and 1983 are awaiting justice. A trial of 55 former naval officers has been dragging on for years and earlier this year the chief prosecutor, Abel Cordoba, claimed judges and the government were deliberately delaying it.
He said he hoped for a verdict by July but the ESMA mega-trial is still dragging on and on Wednesday (August 9) relatives of the victims staged a protest demanding it be speeded up.