Lebanon's army has seized control of the headquarters of an Islamic cleric and arrested dozens of his supporters, following a second day of clashes fuelled by Syria's civil war.

The army said 12 soldiers had been killed in the fighting in Abra, near Sidon, while security sources gave a higher army death toll of 18 and said 20 of Sheikh Ahmed al-Assir's supporters were killed.

The clashes subsided on Monday evening but Assir, who has accused the army of backing the interests of the Shia group Hezbollah, remains at large. Local media reported that he and some supporters had gone into hiding in Ain el Helweh, one of the Palestinian refugee camps nearby, following the battle.

The army is trying to kill or capture him, after accusing him of killing soldiers "in cold blood" on Sunday.

Offices destroyed

Reports from Abra described at least four tanks and several army vehicles ablaze, while abandoned weapons and military clothing were found inside the cleric's headquarters. Many buildings in the complex had been damaged by fire, with Assir's main office said to have been totally destroyed.

Al Jazeera's Nour Samaha, reporting from outside Sidon, earlier said fighting had raged around Abra and the Ain el Helweh camp. Members of the Jund al-Sham and Fatah al-Islam armed groups have joined the battle, she added.

However, roadblocks were lifted and gunfire subsided after the army gained control of Assir's headquarters.

Tensions had been rising in Sidon since the Lebanese Shia group Hezbollah sent fighters into Syria to help recapture a strategic border town controlled by Syrian president Bashar al-Assad's forces.

The army said clashes broke out on Sunday after security forces detained one of Assir's followers. His supporters retaliated by opening fire on an army checkpoint. Army commanders pledged to crush Assir's forces.

Restoring security

"We affirm to all Lebanese that the army is determined to eradicate strife, and will not halt its military operations until security is completely restored to the city," the army said in a statement on Monday.

At least 62 Assir supporters were arrested as soldiers combed the area they had seized, a security source said. One of the men captured had disguised himself as a woman.

Lebanon's commissioner to the military court, Judge Sakr Sakr, said that Assir had been summoned "to be put on trial, along with 123 of his followers, including his brother and Fadl Shaker," a prominent Lebanese singer who abandoned his career to join Assir's ultraconservative group.

"Come and save your people who are being massacred," said an appeal on Assir's Twitter account on Monday.

Assir, whose supporters accuse the army of giving cover to Hezbollah gunmen, called for people across the country to join him and demanded that "honourable" soldiers defect.

Assir has a small group of staunch supporters, believed to be in the hundreds. But many Lebanese Sunnis see him as a militant and troublemaker.

Lebanon's grand mufti, Mohammad Rashid Qabbani, condemned the fighting, saying that there was never a justified reason to attack the armed forces.

But in the northern coastal city of Tripoli, a Sunni stronghold, masked gunmen roamed the streets on motorbikes and fired guns and sound grenades in a show of support for Assir. Other supporters blocked main roads with cement blocks and burning tires.