Globo, the biggest media organisation in South America, has publicly apologised in an editorial for acting as the propaganda arm of Brazil’s military dictatorship between 1964 and 1985.

After nearly 50 years of denying that it had any involvement with the military regime, the Marinho family, the long-standing owners of the company, placed a 1,300-word mea culpa in the errors section of O Globo, its Rio de Janeiro newspaper, at the weekend. The editorial, which was also published online, admitted that the group network regretted not publishing the statement before the anti-government protests erupted in June this year.

During the demonstrations its broadcasting arm, Rede Globo, was targeted by protesters in several cities. Last week the TV station’s headquarters in Sao Paulo was the target of graffiti and manure bombs in a series of protests by the groups Anonymous and Black Bloc.

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“The truth is hard. Globo supported the dictatorship,” the editorial said. “Following many years of internal discussions, the Globo organisation recognises that, in the light of history, this support was a mistake.

“The protesters gave us even more certainty that the internal evaluation was correct and recognising our error was necessary.”

Noticeably, many of Brazil’s major news organisations did not carry the apology. Critics said this is because during military rule, many were also complicit. In its statement, Globo named O Estado de Sao Paulo, Folha de Sao Paulo, Jornal do Brasil and Correio da Manha as news agencies that allegedly agreed with the military intervention.

Rede Record, the second biggest media organisation in Brazil and the main rival to Globo, carried the apology without comment.

Globo justified its actions, saying that at the time the company feared there would be another coup following the military takeover that deposed the democratically elected President Joao Goulart. It said that during the dictatorship in 1964 “[we] always stood firmly against the persecution of left-wing journalists, as is well known. We were keen to employ many of these journalists to write for Globo. When they were called to testify [before the military regime]. We accompanied them personally to prevent them disappearing”. More than 70 people disappeared during the military dictatorship.

The apology cited the views of Robert Morinho, the founder of Globo, who claimed in 1964 that military “intervention was essential to the maintenance of democracy and to contain the outbreak of urban warfare”.

Criticism has come largely from blog writers who call the apology insincere. “I thought it was a text to recognise what had happened and to apologise to all who were expelled from the country, who were arrested, tortured and killed,” Cristina Rodrigues, a freelance journalist, said.

President Dilma Rousseff was among many who, as a result of their opposition to the dictatorship, was tortured and imprisoned.