Three people were killed on Sunday when fighting broke out in the Lebanese coastal city of Sidon between followers of a Sunni Salafist preacher and supporters of the Lebanese Shi'ite movement Hezbollah, a security official said.
The killings underlined the deepening Shi'ite-Sunni divide in the small but regionally important country, mirroring divisions in neighbouring Syria where a Sunni-led revolt against President Bashar al-Assad broke out 20 months ago.
The clash occurred in the Palestinian refugee camp of Ain el-Hilweh on the edge of Sidon, a mostly Sunni city, when followers of Sheikh Ahmed al-Aseer, who adhere to a puritan interpretation of Islam, began removing banners put up by Hezbollah in advance of Ashura, a Shi'ite religious occasion.
Ain el-Helweh, a sprawling camp 40 km (25 miles) south of Beirut, is also home to many Lebanese.
"Tempers flared and the two sides fired automatic rifles at each other," the Lebanese official said, adding that a Lebanese army unit was subsequently dispatched to the area.
Ashura marks the killing of Imam Hussein bin Ali by Ummayyad forces at the battle of Karbala in Iraq in 680, a culmination of a power struggle that ushered in the great Sunni-Shi'ite divide in Islam which still shapes the Middle East's political map.
Prime Minister Najib Mikati, a business tycoon who was a close friend to Assad before the revolt, called upon all parties "to exercise self-restraint and not provoke security incidents in this sensitive point of time".
Tensions have increased in Lebanon against the backdrop of Syrian revolt. Wissam al-Hassan, an anti-Assad Lebanese intelligence commander, was killed by a car bomb in Beirut last month.
Sunni politicians and religious leaders blamed Syria and Hezbollah for the assassination. The two sides deny involvement.
In recent months violence has also flared on several occasions in the northern city of Tripoli, with fighters loyal to opposing sides in Syria's war killing one another in clashes.
Hezbollah, the only Lebanese faction allowed to keep arms since the Taef agreement two decades ago ended a long civil war in the country, has supported Assad as he struggles to survive the Sunni-led revolution against his autocratic rule.
Assad is a member of the minority Alawite sect, an offshoot of Shi'ite Islam that has dominated power in Syria since the 1960s. His father, the late President Hafez al-Assad, played a leading role in setting up Hezbollah in the 1980s, along with Iran's clerical rulers.