Civil Defense teams struggled to put out the flames which engulfed the bookstore owned by Father Ibrahim Srouj in the old Serail neighborhood in the city, turning one of Lebanon's most renowned libraries into rubble.

A security source told AFP that unknown assailants torched Al-Saeh library, destroying "two thirds of some 80,000 books and manuscripts housed there."

"This criminal act poses several questions on the party behind it that aims at damaging coexistence in the city and ruining its reputation,” Ashraf Rifi, former head of the Internal Security Forces said in a statement.

The Lebanese police launched an investigation into the incident.

Former PM Fouad Siniora also condemned the attack, saying the perpetrators only served Lebanon's enemies.

"Whoever did this is doing a service for Lebanon's enemies and the enemies of coexistence ... with the aim of damaging Tripoli's image to resemble a city of extremism," Siniora said in a statement.

Tripoli has seen an escalation of sectarian attacks launched by supporters of the militant groups fighting in Syria to topple the government, resulting in 18 rounds of clashes since the foreign-charged conflict in Syria began in 2011.

SHI/SHI