The incident happened near the coastal town of Naameh, 20km (12 miles) south of Beirut.
One policeman was seriously wounded, security officials said. The official National News Agency said police were investigating what sparked the shooting.
The two policemen were on their way to collect tax arrears from factories in a nearby neighbourhood when they came under fire from guards on the outskirts of the camp, security officials said. The officials declined to give their names because they were not authorised to talk to the media.
The officials said the Palestinian guards who shot the policemen belonged to the Popular Front for the Liberation of Palestine-General Command (PFLP-GC), which is a small, Syrian-backed group that has been fighting Israel for decades.
The PFLP-GC runs an underground base in the hills bordering the town from the east, which is often attacked by Israeli warplanes.
The Lebanese army sent troops to Naameh on Monday as townspeople took to the streets and burned car tyres to protest against the shootings.
The demonstration cut traffic connecting Beirut with southern Lebanon, the witnesses said.
Protesters poured several truckloads of sand on the highway when the army intervened to reopen the road, leading to clashes in which at least two civilians were slightly injured, witnesses said. Traffic has since resumed on the highway.