Earlier on Sunday, Lebanese army soldiers shot dead in northern Lebanon two members of a political party in an alliance against Syrian President Bashar al-Assad.
Many Sunni Muslims in Lebanon's north sympathise with Syria's Sunni-led uprising against Assad and say the Lebanese army is taking orders from Damascus.
Syrian troops were garrisoned in Lebanon, a tiny country still recovering from its own 15-year civil war, until 2005.
"Nine people were wounded in the Tariq al-Jadede district (in southern Beirut)," a security source told Reuters.
A witness said that two Sunni Muslim groups were fighting each other, but it was not clear how the violence had started.
Earlier on Sunday, Sheih Ahmed Abdul Wahid, a Sunni Muslim cleric, and Muhammed Hussein Miraib, both members of the Lebanon-based March 14 political alliance, were shot in their car as they sped through an army checkpoint without stopping.
Residents of the northern region of Akkar blocked off roads and burned tyres to protest against the deaths. The main coastal highway as well as roads in the capital Beirut were also blocked by enraged residents.
The army released a statement confirming the deaths but not giving any information on who was responsible or what led up to the shootings.
"The leadership of the army expresses deep regret for the death of the two victims ... It will immediately form an investigative committee comprised of senior officers and military police under the relevant court," the statement said.
Prime Minister Najib Mikati tried to quell growing tensions. "The government is determined to continue to shoulder its national responsibilities amid this critical period in Lebanon and the region, and it will take all measures necessary to preserve civil peace," he said in a statement.
Some troops pulled out of Akkar to prevent tensions from escalating, security source and residents said.
Khaled Daher, a member of parliament from the Future Movement party, which is part of the March 14 alliance, said the two men had been assassinated.
"If shots were fired at the tyres, we would say there was a mistake. But we consider this a direct targeting from the army," he told Reuters. "Frankly, we do not want to see the army here because it works at the service of the Syrian regime," he said.
Beirut-based political commentator Rami Khouri said recent violence in the northern port of Tripoli had been linked to events in Syria.
"You have tensions in the area going back years but this has been exacerbated by the situation in Syria ... Syria is not the primary factor, but it is related," he said.