Two people were killed and 15 others injured in street battles between pro- and anti-Syrian groups in the Lebanese capital overnight as the spiralling conflict in neighbouring Syria spilled across the border.
Some residents in Beirut kept their children off school following the fighting, which was among the worst the Lebanese capital has seen in four years.
Gunmen fired rocket-propelled grenades and machine guns in battles that lasted more than four hours.
The streets were calmer today, but some shops remained closed.
The violence in the predominantly Sunni neighbourhood of Tariq Jadidah erupted hours after an anti-Syrian cleric and his bodyguard were shot dead at a checkpoint in northern Lebanon, an incident that instantly increased tensions.
Authorities were braced for the possibility of more violence today in the north, where Sunni cleric Sheik Ahmed Abdul-Wahid and his bodyguard were to be buried.
Gunmen carrying automatic rifles shouted for the downfall of the Syrian regime in the cleric's home town of Beireh, where his funeral was to take place later today.
The fighting underlines how the bloodshed in Syria, where President Bashar Assad's regime is cracking down on an uprising against his rule, can fuel violence across the border in Lebanon.
Lebanon has a fragile political faultline precisely over the issue of Syria.
There is an array of die-hard pro-Syrian Lebanese parties and politicians, as well as support for the regime on the street level. There is an equally deep hatred of Mr Assad among other Lebanese who fear Damascus is still calling the shots in their country. The two sides are the legacy of Syria's virtual rule over Lebanon from 1976 to 2005 and its continued influence since.
Today, a gunman in Beireh shouted "Down with Bashar!" and said the Syrian leader was trying to "transfer the crisis to Lebanon".
Lebanon and Syria share a complex web of political and sectarian ties and rivalries, which can easily turn violent. Last week, clashes sparked by the Syrian crisis killed at least eight people and wounded dozens in the northern city of Tripoli.
The revolt in Syria began 15 months ago, and there are fears the unrest will lead to a regional conflagration that could draw in neighbouring countries. The UN estimates the conflict has killed more than 9,000 people since March 2011.
Syrian activists said regime forces killed dozens of people during a raid on the central town of Soran in Hama province yesterday, security officials said.
One activist group, the Britain-based Syrian Observatory for Human Rights, put the death toll at 39, citing a network of sources on the ground. Syria-based activist Mustafa Osso said the figure was more than 20.
The death toll could not be independently confirmed.
A video posted online by activists showed the bodies of five people said to be of the same family who were killed during the shelling of Soran.
Syria is a geographical linchpin in the Middle East, raising the possibility that the crisis there will bleed into other countries.
The circumstances surrounding yesterday's fatal shooting of the Abdul-Wahid, the Sunni cleric, and his bodyguard remained unclear but the state-run National News Agency said they appeared to have been killed by soldiers after their convoy failed to stop at an army checkpoint.
The Lebanese army issued a statement, saying it deeply regretted the incident and that a committee will investigate.
The clashes in Beirut subsided at around 4am local time after anti-Syrian gunmen took control of the headquarters of the pro-Syrian Arab Movement Party.
The fighting was among the most intense fighting in Beirut since May 2008, when gunmen from the Shiite Hezbollah militant group swept through Sunni neighbourhoods after the pro-Western government tried to dismantle the group's telecommunications network.
More than 80 people were killed in the 2008 violence, pushing the country to the brink of civil war.
Also today, Syria's state-run news agency Sana said Mr Assad had issued a presidential decree calling on the country's newly elected parliament to hold its first meeting on Thursday. A parliament speaker is usually elected on the first meeting of a new parliament.
Mr Assad has pointed to the parliamentary elections earlier this month as a sign of his long-promised reforms, but the opposition dismissed the vote as a sham meant to preserve his autocratic rule.