The army officer and three gunmen were killed during attacks in separate parts of the coastal city Sunday night, the Daily Star in Beirut reported.
President Michel Sleiman said the "criminal, terrorist act" came at a time the Lebanese public was looking for protection and stability from the army.
Najib Mikati, the country's caretaker prime minister, called for support of Lebanon's security agencies "and to prevent [anyone] messing with our security."
The attacks were a "cowardly terrorist act," said Sheik Abdel-Amir Qabalan, vice president of the Higher Shiite Council.
Former Prime Minister Saad Hariri called the attackers "a group of misled and irresponsible people," saying Sidon would not be "dragged into a confrontation with the Lebanese army."
The attackers were believed to be supporters of Sheik Ahmad Amir whose fighters clashed with the army in Sidon earlier in the year.