Hundreds of Sunni protesters in Lebanon launched a sit-in Saturday on the main road linking Beirut to Damascus, blocking the passage of fuel tankers to war-torn Syria, an AFP reporter said.

The demonstration, organised by the Association of Muslim Scholars, was staged in the eastern Bekaa Valley to stem fuel supplies intended for forces loyal to Syrian President Bashar al-Assad.

"We are here to obstruct the passage of fuel tankers, which we believe are transporting fuel to propel the killing machines in Syria," said Sheikh Yehia al-Baridi, one of the protest organisers.

About 15 trucks were forced to turn back to the Lebanese capital by the estimated crowd of about 300 people, around 20 of whom were Sunni Muslim sheikhs or clergymen, said the reporter.

The protesters called on the Lebanese authorities to take a stronger position on the Syria crisis, and to stop allowing fuel supplies across the border.

"We are warning the Lebanese state... we as Sunni Muslims believe these tankers carry the fuse to ignite sectarian conflict" in Lebanon, said Baridi.

Though formally neutral on the war in Syria, Lebanon is deeply divided over the conflict, with the Sunni-led March 14 bloc opposed to Assad's regime, and the powerful Shiite movement Hezbollah and its allies supporting it.

Saturday's action was the second time this month that Lebanese protesters have taken matters into their own hands, after dozens cut off two northern border crossings to protest diesel fuel transfers to Syria's regime on February 13.

The violence in Syria has also spilled over into Lebanon on occasion, with cross-border shellings in the north and east, and Syria-related clashes breaking out in flashpoint areas such as Tripoli.

Damascus dominated Lebanon militarily and politically for nearly 30 years, until an international outcry over the assassination in Beirut of prime minister Rafik al-Hariri forced a Syrian troop pullout in 2005.