The altar boy was with other children and neighbours, when the paramilitary group left the headquarters of the ruling Sandinista National Liberation Front (FSLN) and sprayed bullets at them, priest Victor Morales of the diocese of Leon told reporters.
The young man was shot in the chest with a high-caliber firearm and died a few minutes later, the priest said.
Dolmus was an altar boy at the Cathedral of the Assumption of Mary in Leon, classified as a Unesco World Heritage Site.
In addition to the deaths, some 1,000 people had been injured during the civil unrest which began on April 18.
Ortega's government, which has ruled for the past 11 years, blames opposition political groups for the "criminal violence" and denied reports of "riot squads or paramilitary groups linked to the government" who protesters blame for the deaths.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)