At least 15 Lebanese soldiers are now known to have died in clashes between government troops and followers of a Sunni sheikh, who is opposed to the Shia movement Hezbollah and its involvement in the war in Syria.
The clashes which broke out on Sunday between the army and supporters of Ahmad al-Assir continued overnight in Sidon, reported Al Jazeera's Nour Samaha on Monday.
The fighting raged around Abra and the Ain el Helweh Palestinian refugee camp near Sidon, she said.
Members of the Jund al Sham and Fatah al Islam armed groups have joined the battle, she added, reporting from a village several kilometres away from Sidon.
She said clashes had "intensified in both areas mentioned above, with heavy weaponry being used".
"The sound of explosions and gunfire can be heard several kilometres outside Sidon. The army is preventing anyone from leaving the Palestinian camps which are located in Sidon," she said.
"Roads are blocked entering into Sidon from the south, preventing people from reaching Beirut. Checkpoints have been erected, some by the Amal movement [pro-Hezbollah party], to prevent people from going through Sidon - they have been turning people back."
She said masked gunmen had moved to Sidon's seaside road, and there were reports of further sniper attacks.
On Sunday, Assir's supporters had surrounded an army checkpoint in Abra, on the outskirts of Sidon, where a vehicle transporting other supporters of the Sunni cleric had been stopped, a security source said.
|Captain Samer Tanios was one of those killed in Sunday's clashes
The army were fired on, and they returned fire, the source added.
For hours afterwards, intense battles engulfed the district, closing down businesses and forcing residents to flee with their children.
The army called on the country's politicians to take a stand.
Last week, the Sunni sheikh called on supporters to fire on apartments in Abra that he claimed housed Hezbollah members.
Abra is home to a mosque where Assir leads main Friday prayers. The sheikh believes Hezbollah uses the Abra apartments to keep him under surveillance.
His supporters clashed with Hezbollah in Abra last week that left one man dead.
Assir rose to prominence about two years ago over his opposition to Hezbollah and its ally, the Syrian regime of President Bashar al-Assad.
Syria-related tensions have soared in Lebanon, deepening sectarian rifts between Sunnis and Shias.