The Lebanese army announced a state of alert across Sidon on Monday, a day after two attacks upon checkpoints in the southern city left a soldier and four assailants dead, dpa reported.
Troops, backed up by tanks, fanned out across the streets of Sidon. Lebanon's largest Palestinian refugee camp, home to 75,000 people - and reputedly a refuge for those fleeing the law - lies upon Sidon's outskirts.
Meanwhile, the army released details of Sunday evening's attacks. It said that at 9 pm (1900 GMT) on Sunday, soldiers at the al-Awwali Bridge checkpoint fired upon a pedestrian who ran towards them with a hand grenade when asked for his papers.
The grenade exploded, killing the attacker. Two soldiers were wounded. Two companions of the attacker fled.
Shortly afterwards, troops stopped a vehicle at an army checkpoint in the Majdelyoun area. A passenger dismounted and hugged one of the soldiers as he detonated a hand grenade, killing both men.
Soldiers manning the checkpoint then shot and killed the remaining two occupants of the vehicle.
Lebanese President Michael Suleiman condemned the attacks as "criminal, terrorist acts."
Sidon was the scene last June of deadly clashes between the military and followers of Salafist cleric Sheikh Ahmed al-Assir, who is also a staunch supporter of the Syrian opposition and the military.
Majdelyoun is close to al-Assir's Abra stronghold, and the site where the army killed dozens of his supporters during last summer's fighting. The cleric himself has not been seen since.
Lebanese security sources last month said that one of al-Assir's followers was behind the suicide attack against the Iranian embassy in Beirut on November 19, in which 25 people were killed, including one Iranian diplomat.