Buenos Aires (Reuters) - Tens of thousands of Argentines marched through the rain in front of the presidential palace on Sunday to demand answers in the case of a young protester who went missing two months ago. Opposition and human rights groups believe state security forces took Santiago Maldonado, a 28-year-old craftsman, and allege President Mauricio Macri s government is covering up information on his whereabouts. Maldonado was last seen during an Aug. 1 operation by the National Gendarmerie to end a land occupation in Chubut province by the indigenous Mapuche.  My intuition tells me the Gendarmerie have him. Where, I don t know,  Maldonado s brother, Sergio Maldonado, told reporters at the rally in downtown Buenos Aires. Protesters marched carrying images of Maldonado s bearded face and signs that read  may he appear alive now.  Potential cases of abuse by security forces are sensitive in Argentina, where the 1976-1983 military dictatorship secretly detained, tortured and killed people in clandestine prisons. Rights groups say up to 30,000 people  disappeared.  Macri s government has said that there is no evidence that shows security forces detained Maldonado. 