At least two people have been killed in the Lebanese city of Tripoli as clashes erupted between Sunni Muslims and members of the Alawite minority.
The violence began overnight as armed groups from an Alawite enclave clashed with Sunni fighters, after security forces arrested a Sunni cleric who was reportedly helping Syrian refugees.
Tensions in the northern port city have mounted since Syria's uprising began.
Similar recent clashes have highlighted how tensions can spill over to Lebanon.
Syrian President Bashar al-Assad is an Alawite.
In February at least two people were killed in Tripoli as supporters and opponents of Mr Assad clashed.
But the city's Alawite minority has fought with its Sunni neighbours on several occasions since the 1975-1990 Lebanese civil war.
The latest fighting began last night after the Lebanese security forces arrested a Sunni cleric, named as Shadi al-Moulawi, on charges of aiding a terrorist organisation.
His supporters said he had been helping Syrian refugees.
Two rocket-propelled grenades fell on the Bab Tabbaneh neighbourhood of Tripoli and reports say explosions were heard across the city.
"The clashes peaked at dawn. The sound of gunfire is still echoing in the city," a security official said, quoted by Reuters.
Lebanese Army units were deployed between the rival neighbourhoods, and the army said reinforcements were on their way.
A soldier was among the people killed in the fighting.
Tripoli is dominated by Sunni Muslims, who support the anti-Assad uprising in Syria.
Members of the minority Alawite sect - an offshoot of Shia Islam - occupy key positions in the Syrian government and security forces.
Syria's majority Sunni community has been at the forefront of the revolt against the president and borne the brunt of the state's crackdown.
The BBC's Jonathan Head in Istanbul says that community leaders in Lebanon have repeatedly warned of the possibility that the violence in Syria would spill over the border. Lebanon is already hosting thousands of Syrian refugees.
In recent years the fear of renewed civil war has helped persuade the various Lebanese factions to put aside their historic disagreements, but Syria is proving a very tough test for a country with so many sectarian divisions of its own, our correspondent says.