Hundreds of youths on motorcycles waving their party and religious flags gathered in downtown Beirut chanting "Shi'ites, Shi'ites" and setting fire to tyres. They hurled stones and fireworks at security forces standing nearby, witnesses said.
Ignoring calls for restraint by politicians, the youths tried to break a security cordon to storm the square where demonstrators have set up tents as part of an anti-government protest that has been going on for weeks.
In the vicinity of the area close to a main road that links the capital's eastern and western sections, scores of youths had burnt tyres, smashed office buildings and torched several cars, live coverage by local television stations showed.
The protesters camped in the square have been targeted by Shi'ite groups in the past angered by chants against their political leaders, although Tuesday's violence was of an overtly sectarian nature.
The video, which inflamed passions in a country where sectarian divisions run deep, was purportedly made by an expatriate Lebanese Sunni from the northern city of Tripoli and released on social media.
In the predominately Sunni populated city of Saida in southern Lebanon, groups of masked youths stormed into a main square where they set ablaze several tents set up by demonstrators who have camped for weeks, local television stations said.
Lebanon has been gripped by a historic wave of protests since Oct. 17 leading to the resignation of Saad al-Hariri as prime minister, amid anger at the government's failure to address the country's worst economic crisis since the 1975-1990 civil war.
More than seven weeks since Hariri quit, politicians have been unable to agree on a new administration despite the deepening financial crisis.
The impasse took a violent turn at the weekend when Beirut was clouded in tear gas as security forces clashed with protesters who blame the ruling elite for corruption and bad governance. Dozens were wounded.