The army said in a statement that eight soldiers and two civilians were killed in Saturday's attack in the jungles of central Peru.
The earlier death toll of seven rose after forces found the bodies of soldiers who had previously been reported as missing.
The army said guerrillas attacked a military convoy that was transporting election material and forces tasked with guarding polling places in the central Junin region.
Authorities blamed remnants of the Shining Path communist guerrilla group, which was largely crushed in the 1990s, but still has members hiding in the jungle.
The army said attackers first struck at Hatun Asha, located in a jungle zone considered a stronghold of the guerrillas and a major coca-producing area.
In a second attack, they targeted a military ship on the Apurimac River in the south, wounding two soldiers, authorities said. President Ollanta Humala condemned the "demented" violence.
"Terrorism and those who collude with it have no place in our society or in our family," he said on Saturday. Some 23 million Peruvians voted Sunday for a new president and members of congress.
Conservative candidate Keiko Fujimori topped the ballot and must face a runoff vote against her center-right rival Pedro Pablo Kuczynski.
Keiko Fujimori's father Alberto Fujimori waged a fierce conflict against the Shining Path when he was president from 1990 to 2000.
Around 69,000 people were killed between 1980 and 2000 in the conflict with the Shining Path, according to the country's Truth and Reconciliation commission.