Farmers in the central region of Gorongosa region last week said they found the bodies, some of which appeared to have been recently buried while others had decomposed, in a former open-pit mine.
The area has been a hotspot of rising tensions between government forces and Renamo, the rebel group that fought a 16-year war against the state ending in 1992 and later became an opposition party.
“The Mozambican authorities should urgently act on reports of a mass grave,” Zenaida Machado, Africa researcher at Human Rights Watch, said in statement.
“They should immediately launch a credible and transparent investigation.
“Denying the existence of the grave without appropriate investigations only serves to raise suspicions about what happened.”
The site of the alleged grave has been cordoned off by government forces.
“We have found no evidence, we have not located the alleged mass grave and we have not been able to identify and talk with the farmers who supposedly found it,” a police spokesman said Tuesday.
Renamo spokesman Antonio Muchanga said its members were routinely targeted by the security forces, claiming 50 had been murdered in recent months.
Renamo rejected the results of 2014 elections, which were won by Frelimo, the movement which led the country to independence from Portugal in 1975 and has ruled ever since.
Clashes between security forces and rebels have intensified since December.
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