Supporters of the Hezbollah and Amal movements fought with protesters at a roadblock on a main road in Beirut on Tuesday, pulling down their tents and leading police to step in.
The men were chanting in support of Hezbollah leader Hassan Nasrallah and parliament Speaker Nabih Berri, head of Amal.
The incident started when the men pushed their way into the protest camp set up by demonstrators on the Ring Bridge in central Beirut, trying to make them open the road.
They then made their way to the nearby Martyrs’ Square – the main camp of protesters who had taken to the streets since October 17.
The men, many of them wielding batons, rampaged through the camp, torching tents, smashing plastic chairs and chasing away protesters.
The rampage marked a violent turning point in Lebanon's protests, which have called for the resignation of the government and the overthrow of the political class that has dominated the country since its 1975-1990 civil war.
The men who attacked the protesters initially presented themselves as disgruntled residents, frustrated that the protests have paralyzed the country. But later they could be heard making pro-Hezbollah and Amal chants.
The counter-protesters lashed out at reporters covering the melee, kicking some of them and reaching for their cameras.
It was unclear how many people were wounded. Fights broke out among some counter-protesters and security forces could be seen beating some people with batons.
Security forces later fired tear gas to disperse them, but only after they had destroyed and set fire to several tents.
The protests have thus far been largely peaceful, with only occasional minor scuffles with security forces trying to open the roads.
Hezbollah supporters had clashed with protesters and security forces in the nearby Riad al-Solh Square last week.
People from all sects and walks of life have flooded the streets for 13 days, furious at a sectarian ruling elite they accuse of plundering state resources for personal gain.
They are demanding the resignation of the government and the ruling elite.
Later Wednesday, Prime Minister Saad Hariri submitted his resignation to President Michel Aoun.
Protesters have resorted to blocking roads throughout the capital and other major arteries in the country to press their demands.
