Supporters of the Hezbollah and Amal movements attacked Lebanese anti-government protesters in Beirut on Monday, with army reinforcements intervening to diffuse tensions.
Shortly before midnight on Sunday, men on foot and scooters arrived at a roadblock set up by anti-graft protesters across a key street in the capital, local television showed.
They shouted insults and chanted in support of the chiefs of the Shi'ite movements Hezbollah and Amal, before briefly breaking through and attacking some demonstrators.
Those at the roadblock chanted "peaceful, peaceful", as the security forces and army reinforcements deployed in a thick line between both sides in the early hours of Monday morning.
The counter-protesters also headed to a main nerve center of protests nearby and destroyed tents there, a local television channel said.
The tensions came after a peaceful day of demonstrations, more than a month into a spontaneous nationwide street movement against the political elite.
On Saturday, Lebanese security forces briefly detained five youths, including three minors, for allegedly pulling down a sign for President Michel Aoun's political party in the town of Hammana east of Beirut, sparking outrage on social media.
Security forces released them after midnight, the Committee of Lawyers for the Defense of Protesters said.
The army said two of the children were 15 years old and the third was 12.
"Down with the regime that arrests children," a Twitter user said.
"When a 12-year-old child manages to shake the state's throne, you know the state is corrupt," another wrote.
During the first month of demonstrations, security forces arrested 300 people including 12 minors who were released within 24 to 48 hours, according to the lawyers' committee.
But 11 people -- including two minors -- remain in detention accused of attacking a hotel in the southern city of Tyre during the first week of the uprising.
The demonstrators managed to bring down the government less than two weeks into the protests, but it remains in a caretaker capacity and no new cabinet has since been formed.
Late Sunday, protesters blocked major roads in several parts of the country and called for a general strike the following day in protest at the lack of progress in forming a fresh government.
Earlier, hundreds had gathered in protest centers in Beirut, the northern city of Tripoli and in Tyre.
In Beirut's Martyrs' Square, hundreds of women and men demanded their rights, some waving the national red and white flag or chanting "Revolution, Revolution!"
Lebanon's protests have brought together people of all ages from across the political spectrum, tired of what they describe as sectarian politics three decades after a civil war.
In the latest show of unity, a festive mood had reigned Sunday afternoon as Lebanese came together in public spaces across the country on the second day of the weekend.
North of the capital women prepared traditional salads to share, while a group of men danced on a beach south of the city, state television footage showed.
The Free Patriotic Movement party that Aoun founded is now led by his son-in-law, outgoing foreign minister Gibran Bassil, one of the most reviled figures in the protests.
Hezbollah is the only party not to have disarmed after the 1975-1990 civil war and plays a key role in Lebanese politics.