Tuesday has been declared a day of mourning in Lebanon after 48 hours of fighting in the southern port city of Sidon left at least 16 soldiers dead.
The sectarian clashes, which are linked to the war in neighbouring Syria, are the deadliest since the conflict erupted two years ago.
Gunfights between the army and Sunni Muslim groups extended into Monday night after soldiers stormed a complex holding gunmen loyal to a radical Islamist cleric, Ahmed al-Assir.
He remains at large, but dozens of his supporters were arrested and more than 20 bodies were later pulled out, according to one witness.
Tension that had long been simmering boiled over last week. The violence pits Sunni and Shi’ite Muslim fighters, at odds over the Syrian conflict, against each other.
There are fears that Lebanon, still scarred by its own civil war of three decades ago, could be dragged back into similar strife.
On Monday the clashes spread to Tripoli in the north, where gunmen fired on the army and blocked roads. Two soldiers and three militants were wounded.
Beirut is also on high alert. Reports say some hardline Sunni mosques have called for jihad, or holy war, in support of the cleric whose complex was stormed.