Published the day before the 41st anniversary of the coup, the letter said the overthrow of socialist president Salvador Allende set the South American country on the path to "security" and "progress."
"We salute all Chileans on the foundational date of 21st-century Chile," said the ad in daily newspaper La Tercera, which was signed by 20 organizations of soldiers, police and "martyrs' widows" whose military husbands were killed.
The letter condemned the trials of military officers accused of crimes during Pinochet's "dirty war" against leftist opponents, when 3,200 people were killed and 38,000 tortured, according to government figures.
"We will not cease publicizing this abnormal situation, which damages the concept of justice and Chileans' solidarity."
About 60 former military members are currently serving prison sentences for violating human rights during the dictatorship.
The September 11, 1973 coup remains deeply divisive in Chile, where Pinochet still has fervent supporters 24 years after the return to democracy and eight years after his death at age 91.