They were killed by "armed men who have not yet been identified" in Bakassi's Isanguele district, Minister Marafa Hamidu Yaya told reporters as votes were being counted across the country.
"These brave elements of our security forces were on a mission to secure the electoral process," the minister said.
"All steps are being taken to find and apprehend their killers."
About seven million Cameroonians were eligible to vote for a president Sunday with incumbent Paul Biya almost assured of extending his 29-year rule amid signs of apathy in a ballot the opposition termed a "mess".
Several groups, often armed, operate in the coastal Bakassi peninsula, carrying out assaults and kidnappings the authorities blame on pirates.
The Bakassi peninsula, a marshy coastal region rich in fish stocks and believed to have substantial oil reserves, was at the centre of a territorial dispute between Nigeria and Cameroon for 15 years.
In August 2008, the International Court of Justice ruled in favour of Cameroon's claim to the territory.
