On the tape, which TV Globo aired early on Sunday, a member of the prison gang known as the First Command of the Capital, or PCC by its Portuguese initials, criticised the dire conditions of Brazil's prison system.
Guilherme Portanova, a reporter for the TV Globo channel, and his assistant, Alexandre Coelho Calado, were abducted by three armed men on Saturday at a bakery near the station's studio in Sao Paulo, Globo said in a statement.
Calado was later released in front of the studio with a recording that his kidnappers demanded be aired by Globo, Brazil's most influential TV network.
Portanova, who frequently covers the crime stories, has not been released.
The statement was read by a suspected PCC member who was standing in front of a white cloth with the words "Peace and Justice".
Globo said the video also contained images of weapons such as grenades, rifles and submachine guns.
"We want a prison system with humane conditions, not a bankrupt, inhumane system in which we are subjected to innumerous humiliations and beatings," the statement read.
"The Brazilian penal system is, in reality, a true human dump, where human beings are thrown as if they were animals."
The kidnappings capped a week of violence in which the PCC attacked police and torched buses, banks and government buildings in cities around the state.
At least six suspects have been killed by police and at least 28 have been arrested since the attacks began last Sunday.
The latest unrest marks the third wave of PCC violence in four months in and around Sao Paulo.
The PCC, which was born in Sao Paulo's overcrowded prison system in 1993, called the latest attacks to demand temporary releases for this weekend's Father's Day holiday.
More than 10,000 inmates were released for the holiday, putting the state's forces on alert.