Lebanon's third-largest city of Sidon was turned into a battle zone Monday as the military fought heavily armed followers of an extremist Sunni Muslim cleric holed up in a mosque.
Residents of the southern port city fled machine-gun fire and grenade explosions that shook the coastal area in one of the deadliest rounds of violence, seen as a test of the weak government's ability to contain the furies unleashed by the civil war in neighbouring Syria.
Official reports said at least 16 soldiers were killed and 50 were wounded in two days of clashes with armed followers of Ahmad al-Assir, a maverick Sunni sheik whose rapid rise is a sign of the deep frustration among many Lebanese who resent the ascendancy of Shiites to power, led by the militant group Hezbollah. More than 20 of Mr. al-Assir's supporters were killed, according to a security official who spoke on condition of anonymity because he wasn't authorized to talk to reporters.
The fierce battle that Mr. al-Assir's fighters were putting up showed how aggressive Sunni extremists have grown in Lebanon, building on anger not only at Syria's regime but also its allies in Hezbollah.
"Sidon is a war zone," said Nabil Azzam, a resident who returned briefly Monday to check on his home after having fled with his family a day earlier. "This is the result of all the sectarian rhetoric that has been building because of the war in Syria. It was bound to happen," he said by telephone, a conversation interrupted by a burst of gunfire.
Snipers allied with Mr. al-Assir took over rooftops, terrorizing civilians. Many were asking to be evacuated from the heavily populated neighbourhood around the Bilal bin Rabah mosque, where the cleric preaches and where the fighting has been concentrated.
The military appealed to the gunmen to turn themselves in, vowing to continue its operations "until security is totally restored." By evening, the army had stormed the mosque complex, though not the mosque itself.
There was no sign of Mr. al-Assir and it was unclear if he was in the mosque or had managed to escape. Reuters reported that his Twitter account posted an appeal on Monday saying, "Come and save your people who are being massacred."
The scenes of soldiers aiming at gunmen holed up in residential buildings and armoured personnel vehicles deployed in the streets evoked memories of Lebanon's 1975-90 civil war, which eventually splintered the army along sectarian lines.
"It's the memory of this destructive war that remains as a restraining force – for now," said Fawaz A. Gerges, director of the Middle East Centre at the London School of Economics.
Syria's civil war has been bleeding into Lebanon for the past year, following similar sectarian lines of Sunni and Shiite camps. Overstretched and outgunned by militias, the military has struggled on multiple fronts in the eastern Bekaa Valley and the northern city of Tripoli as armed factions fought street battles that often lasted several days.
In many cases, soldiers stood by helplessly and watched the violence, but on Monday the army moved against Mr. al-Assir after his followers opened fire on an army checkpoint without provocation.
Mr. al-Assir, a 45-year-old bearded cleric who supports the overwhelmingly Sunni rebels fighting to topple Syrian President Bashar al-Assad, is an unlikely figure to challenge the Lebanese army. Few had heard of him until last year, when he began agitating for Hezbollah to disarm, taking advantage of the deep frustration among Lebanon's Sunnis and a political void on the Sunni street following the 2005 assassination of former Prime Minister Rafik Hariri, a powerful Sunni leader.
Mr. al-Assir kept local and international media entertained by pulling stunts such as riding his bicycle and getting his hair cut in public, and caused a stir in February when he and hundreds of his bearded supporters arrived in buses at a ski resort in the Christian heartland.