Three Lebanese were killed in the southern port city of Sidon in a gunbattle on Sunday between supporters of the Shiite group Hezbollah and a hardline Sunni cleric, a security official said.
"Three people were killed and seven injured in clashes between supporters of Sheikh Ahmed al-Assir and Hezbollah near a Palestinian refugee camp in Sidon. One of the injured was a representative for Hezbollah for the area," the source told AFP on condition of anonymity, updating an earlier toll.
An AFP correspondent in Sidon said Assir had given an ultimatum to Hezbollah supporters in the city to take down all posters promoting the Shiite militia, but that they refused.
Supporters of the cleric then ripped down a poster of Hezbollah chief Hassan Nasrallah, a staunch ally of Syria's President Bashar al-Assad, and clashes ensued.
Unknown before the outbreak of the uprising in neighbouring Syria last year, Assir rose to prominence for his outspoken opposition to the Assad regime and his calls for disarming Hezbollah, the strongest military force in the country.
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"There will be no security compromises after today. The army will open fire on anyone holding a weapon," Interior Minister Marwan Charbel said during an emergency security meeting with local security officials in Sidon.
The statement came after Prime Minister Najib Mikati called on Lebanese to "remain calm and execute restraint at this critical and delicate juncture".
The premier also requested the army and security agencies take prompt measures to bring the situation under control and arrest those behind the violence.
Lebanon's opposition coalition has accused Mikati, a prominent Sunni figure, of complacency in leading a Hezbollah-controlled government.
The Syrian revolt has deepened political and religious divisions in Lebanon, which is split between supporters and opponents of the Damascus regime dominated by Alawites, an offshoot of Shiite Islam.