It's something that had never been done before in the Republic of Congo.
Security forces have started to vote in the town of Dolisie, ahead of Wednesday's presidential election.
12 000 soldiers and police officers casted their vote.
According to the Congolese authorities, this would free up security forces, and allow them to focus on securing polling stations on Wednesday.
But most of the opposition candidates have pointed the move as an fraud attempt, that would give President Denis Sassou Nguesso a headstart in the election.
Bertrand Menier, of the "Mouvement Ras-le-bol" (Fed-up Movement), said the voters lists did not dissociate members of the military from civilians.
"We fear military personnel might vote twice, given that their names are on both civilian and military lists" he said to Africanews.
The National Electoral Commission has said the security forces ballots will be counted at the same time as those of the civilians.
Denis Sassou Nguesso, 77, who has spent 36 years at the head of Congo, now faces 6 candidates, amongst which two of his former ministers, Mathias Dzon and Guy-Brice Parfait Kolélas.