Guns and rocket fire rattled the port city 40 km (28 miles) south of Beirut that has been on edge since violence erupted there last week between Sunni and Shiite Muslim fighters, at odds over the Syrian conflict.
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Lebanese authorities have been trying to quell the fighting and Sunday's clashes began when police arrested a follower of hardliner cleric Sheikh Ahmed al-Assir at a checkpoint, sources in the city told Reuters.
Protesters in with Hezbollah supporters. Archive. (Photo: AP)
Other Assir backers attacked security forces in the city in retaliation and called on their supporters to come out onto the streets across Lebanon.
Six soldiers were killed and 19 wounded, a security source said. At least two Sunni gunmen were killed and 13 wounded, he added.
Syria's conflict, pitting mainly Sunni rebels against President Bashar Assad, an Alawite from an offshoot of Shiite Islam, has strained fragile sectarian relations in Lebanon and triggered clashes in Sidon and other cities.
Tensions have mounted since the Lebanese Shiite militant group Hezbollah sent fighters over the border to back Assad.
The Lebanese army launched a crackdown in Sidon and sent troops to potential hot spots across Beirut, aiming to stop clashes spreading to the capital.
Beirut protest against Hezbollah (Photo: AFP)
"The army has tried for months to keep Lebanon away from the problems of Syria, and it ignored repeated requests for it to clamp down on Sheikh Ahmed al-Assir's group," the military command said in a statement.
"But what has happened today has gone beyond all expectations. The army was attacked in cold blood in an attempt to light the fuse in Sidon, just as was done in 1975," it said, referring to the year that Lebanon's own 15-year civil war began.
On Friday, security forces found an unexploded rocket hours after another rocket hit a mountain town outside Beirut.
Clashes have also rocked the eastern region in Lebanon's Bekaa Valley, which is home to a large Shiite population and also Sunni backers of the Syrian opposition.
Assir, whose supporters accuse the army of giving cover to Hezbollah gunmen, called for people across the country to join him and demanded that "honorable" soldiers defect.
"We call on all our supporters in all areas who are able to come and protect our religion, our women, and our honor To all noble people in the army, Sunni or not Sunni, you must leave the army" he said in a YouTube video published on Sunday, as the sound of guns and mortar fire echoed in the distance.
The long-bearded sheikh wore a black bullet vest and appeared to be holding a gun.
"We are under attack by the Lebanese army, which is Iranian and sectarian and from Hassan Nasrallah's thugs," he said, referring to the leader of Hezbollah and its regional patron Iran.
As fighting continued into the evening, hard-line supporters of Assir blocked roads in the capital and in the largely Sunni city of Tripoli on the northern coast.
Residents in the northern Lebanese city of Tripoli said they saw masked gunmen blocking roads with burning tires and heard sporadic gunshots, apparently fired in solidarity with Assir.
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