A security source described Bilal Mezher as an "important catch", close to Sirajeddine Zureiqat, head of the Abdullah Azzam Brigades. Mezher, who is Lebanese, was arrested in a special operation in Naameh area south of Beirut.
Arab Muslim youths say IS, Qaeda distort Islam: poll
"This special operation is part of a wider campaign to arrest terrorists and all those linked to terrorist groups," the source told Reuters.
The Abdullah Azzam group, named after a Palestinian regarded as the one-time spiritual mentor of al Qaeda chief Osama bin Laden, has claimed attacks across the region including a double suicide assault on Iran's Beirut embassy in 2014.
The Azzam Brigades are designated by the US State Department as a foreign terrorist organization.
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