Two people were killed and at least three injured in clashes in a town south of the Lebanese capital, Beirut, on Thursday (August 28) evening.
That's according to the state news agency and a security source, who both said gunfire was exchanged.
Several political parties have urged calm.
Lebanon is on edge, following an economic crisis, lockdowns and then a devastating blast at the port earlier this month.
There are fears the clashes could escalate and compound those crises amid a political vacuum.
Local media showed footage of heavy gunfire and buildings on fire.
The Lebanese army said on its Twitter account that it had deployed units to Khaldeh to "control the situation" and restore calm.
According to multiple official sources, the confrontation erupted after a row a few days ago over a banner marking the Shi'ite Muslim Ashoura religious ritual in an area where both Shi'ites and Sunnis live.
The Lebanese army said it had arrested four people in connection with the clashes.