Dozens of people were wounded in overnight clashes between security forces and supporters of Lebanon's two Shiite political parties, the civil defence said on Tuesday.
Young supporters of Hezbollah and Amal tried to attack the main anti-government protest camp in central Beirut, an AFP photographer reported.
The angry young men, who arrived on foot and scooters, threw stones and fireworks towards the anti-riot police trying to prevent them from entering the largely empty main square.
They also set fire to several cars.
The security forces responded with teargas and a water cannon.
The clashes seem to have been provoked by a video shared online that the counter-protesters deemed offensive to their faith.
Civil defence said 23 people were taken to hospital, while 43 were treated at the scene. It was not immediately clear from which side the wounded were.
In the southern city of Sidon, young protesters also attacked a protest camp during the night, destroying several tents, an AFP correspondent reported.
Young Hezbollah and Amal partisans have on several occasions attacked or tried to storm anti-government demonstrators over the past two months.
The nationwide protests that broke out on October 17 have been largely peaceful, with the exception of the alleged use of excessive force by security forces against anti-government protesters over the weekend.
Prime minister Saad Hariri stepped down under street pressure on October 29.
But no consensus has yet been reached on a replacement and parliamentary consultations to name a new premier have been twice postponed.