Leftist Shining Path militants killed at least 16 people, including two children, in a remote region of Peru and burned some of the bodies beyond recognition, according to the military.
Peru's interim president said on Tuesday there will be "no impunity" for the authors of the massacre in the jungle region known for cocaine production, which authorities attribute to a dissident faction of Shining Path rebels.
"We are doing all we can to deploy the police and the military in a way that we can efficiently combat this plague," interim President Francisco Sagasti told reporters. "We know this is a rough terrain with many ravines that the narcoterrorists know very well."
Pamphlets encouraging Peruvians to refrain from voting in the June 6 presidential election were found at the site of Sunday's massacre, the joint command of Peru's armed forces said in a statement.
The military called the murders "an act of genocide" and said the Shining Path had previously labeled such attacks a form of "social cleansing." The statement assured Peruvians of "a secure electoral process."
The incident took place in a region called Valle de los Rios Apurimac, Ene y Mantaro (VRAEM), which produces 75% of the South American nation's cocaine, according to authorities.
The VRAEM, a mountainous region the size of Puerto Rico, is the center of constant operations by the security forces against remnants of the Shining Path, which police say acts as "bodyguards" for drug traffickers.
The Maoist rebel group launched one of the deadliest internal conflicts in Latin America in the 1980s. An estimated 69,000 people were killed, according to a truth commission.
The Shining Path began to fade in the early 1990s after the jailing of founder Abimael Guzman and has since developed ties to narco-traffickers.
Peru is set to hold elections in less than two weeks, pitting socialist Pedro Castillo against conservative Keiko Fujimori. Both candidates denounced the attack.
"I urge the police to investigate immediately and find those responsible," Castillo told reporters.
Fujimori said, "Terrorist groups that want to generate fear, we must not allow them."
The United Nations condemned the "murder" of the people and expressed solidarity with the victims and their families.
"In the framework of the ongoing electoral process, we call on all actors to act responsibly, avoiding hate speech that increases tension," its Lima office said in a statement.