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In Beirut, several hundred turned out to mourn Ahmad Jamal Omeirat, the al-Ahbash follower. Friends at the funeral said he was just 17 years old. Mr. al-Fakhani, the movement’s spokesman, said Omeirat was a student who was killed by a gunshot to the chest.

Lebanese President Michel Suleiman condemned the fighting and urged authorities to arrest those who were behind the shooting. The emir of Qatar, who brokered the Doha Agreement, which put an end to the May 2008 fighting, called Mr. Suleiman on Tuesday night to offer any assistance, the president’s office said.

Lebanon’s government is an uneasy coalition of a Western-backed bloc and Hezbollah, which in just a few years has gained so much political power it now has a virtual veto over government decisions.

Al-Ahbash, or the Association of Islamic Charitable Projects, is a deeply conservative Muslim group and a rival to many other Sunni groups in the country, including the prime minister’s Future movement.

The group’s name rose to prominence in the wake of the Hariri assassination. Two senior officials from the group were detained for about four years on suspicion of involvement in the killing but later were released.

Like Hezbollah, al-Ahbash is pro-Syrian. They have feuded in the past over theological differences but were political allies whose candidates ran on the same lists during the 2009 parliamentary elections.