Assir and his supporters recently participated in a two-day battle with soldiers that left 18 soldiers and at least 28 militants dead, The (Beirut) Daily Star reported.
Demonstrations were held in cities throughout Lebanon to back Assir, who has since been in hiding.
Protesters also condemned a video of abuse by Lebanese soldiers, the news report said.
Mufti Sheik Salim Sousan of Sidon called for an investigation into the video that allegedly showed troops beating a detained civilian.
"We call for a fair, objective, legitimate and transparent investigation with some military groups that tarnished the image of the military institution by the practice of beating, abuse and ill-treatment," Sousan told worshipers at Zaatari Mosque in Sidon's main Nejmeh Square.