Street battles between pro- and anti-Syrian groups in Beirut killed two people, a security official said, sparking concerns of a wider conflict in Lebanon.
The violence erupted hours after reports emerged that troops had shot dead Sheikh Ahmad Abdul Wahid, a prominent anti-Syria Sunni cleric, when his convoy failed to stop at a checkpoint in north Lebanon on Sunday. Another cleric in the car was also killed.
Washington expressed concern over the killings and urged restraint.
"We welcome the commitment of the Lebanese government and the Lebanese Armed Forces to conduct a swift and transparent investigation of the shooting incident," US State Department deputy spokesman Mark Toner said.
"And we call on all parties to exercise restraint and respect for Lebanon's security and stability," he said.
UN chief Ban Ki-moon also called "on all parties to make every effort to restore calm".
The UN head said Derek Plumbly, Special Coordinator for Lebanon, was "engaged on the ground, encouraging all concerned to work for sustained calm and stability in the country".
He also stressed the "need for Lebanon's continuing stability", and encouraged "all Lebanese parties to strengthen their efforts to date to overcome any emerging challenges on the ground".
In Brussels, EU foreign policy chief Catherine Ashton said she was "deeply concerned at the outbreaks of violence in Tripoli and Beirut".
"These events must be duly investigated and followed by Lebanon's law enforcement authorities," she said.
Protesters blocked roads and burned tyres in the northern region of Akkar following the killing of the clerics, and by evening the unrest had spread to the capital.
"During the night, groups of young men cut off the road in the Tareek el-Jdideh district and street battles followed," the security official said, requesting anonymity.
"Two people were killed and 18 were wounded," he said, adding machineguns had been fired and that the fighting had raged until about 3:00 am (2400 GMT).
An office housing a small pro-Syrian party in Tareek el-Jdideh, a mainly Sunni Muslim neighbourhood of west Beirut, was torched by partisans of ex-premier Saad Hariri's Future Movement and the facade of the building was riddled with bullets.
Several motorcycles and cars parked on the street below were burned.
Calm had been restored by daylight following appeals from Prime Minister Najib Mikati and other politicians.
However tension was palpable in the capital, where residents fear a repeat of sectarian clashes like those that left some 100 people dead in 2008 and brought the country close to civil war.
"I've had enough... of war," said Amal Khattab, a 40-year-old teacher and mother of two who lives in Tareek el-Jdideh.
"My children were terrified last night," she added, crying. "I can't spend another minute in this country."
Wahid's funeral was held on Monday in his hometown of El Bireh, in the northern Akkar region, where many businesses and shops were closed after Sunni religious leaders called for three days of mourning.
"We want a fair trial, and we want the killers of Sheikh Ahmad Abdul Wahid to be executed," Future Movement MP Khaled al-Daher said at the funeral.
A judicial official said military police were questioning 21 soldiers, including three officers, in relation to the death of the two clerics.
Their killing followed a week of intermittent clashes between Sunnis hostile to the Syrian regime and Alawites who support it which left 10 people dead in the northern port city of Tripoli.
Late on Monday, two rockets were fired in Tripoli, one landing in a predominantly Alawite neighbourhood and the other between Jabal Mohsen and Bab al-Tebbaneh, which is mainly Sunni, a security official said.
The violence has highlighted a deep split between Lebanon's political parties, where the opposition backs those leading the uprising against Assad while a ruling coalition led by Shiite militant group Hezbollah supports the Damascus regime.
The Sunni-led opposition has accused Assad of seeking to sow chaos in Lebanon in order to relieve the pressure on his embattled regime.
Reflecting mounting fears of an escalation, Bahrain, Kuwait, Qatar and the United Arab Emirates urged their citizens to avoid travel to Lebanon.
Sheikh Nabil Kaouk, a high-ranking member of Hezbollah, hit out at the opposition on Monday accusing it of transforming the north of the country into a rear base for Syrian rebels.
"The opposition has intentionally dragged Lebanon into the Syrian crisis and transformed into into a corridor and base for armed Syrians," he told a rally in the eastern city of Baalbek.
Since the outbreak of the revolt in Syria in March of last year, thousands of Syrians fleeing the unrest in their country as well as activists have sought refuge in northern Lebanon.
Syria long held sway in Lebanese politics and had troops stationed in the country for 29 years until it was forced to withdraw them in 2005 following the assassination of ex-premier Rafiq Hariri.
It has denied accusations it was involved in his killing.