A masked gunman opened fire at the entrance of a Palestinian refugee camp in Lebanon Monday, seriously wounding two guerrillas, a spokesman for the guerrilla group said. Fouad Osman, a member of the Popular Front for the Liberation of Palestine, said the two men were guarding the northern entrance of the sprawling camp near this southern Lebanese port city when the gunman using an assault rifle opened fire. The two men were rushed to a hospital for treatment. The cause of the attack was not known and was being investigated, Osman said. Ein el-Hilweh, with a population of 75,000, is the largest of 12 Palestinian refugee camps in Lebanon. Nearly all Palestinian factions have offices in the camp, where political assassinations, vendettas and other violent incidents are common.