The violence is a reflection of Lebanon's political dysfunction, a legacy of years of civil war when the country became a proxy battleground for other nations. Lebanon walks a fragile fault line over Syria, which had troops on the ground here for nearly 30 years until 2005 and still has strong ties to Lebanon's security services.
To many observers, it was only a matter of time before the violence in Syria infected Lebanon. The U.N. estimates the Syrian conflict has killed more than 9,000 people since March 2011, when President Bashar Assad started cracking down on a popular uprising.
"The Syrian regime is seeking to sow chaos in Lebanon!" Khaled Daher, a Sunni member of parliament, said during a fiery speech Monday at the funeral for Sheik Ahmed Abdul-Wahid, the slain anti-Syrian cleric. "But we will not be scared."
Daher stood surrounded by Sunni clerics and armed gunmen in the northern village of Bireh, Abdul-Wahid's hometown. Syria is visible across the border, on the other side of a green valley dotted with homes and farms.
Gunmen shouting "Down with Bashar!" roamed the streets ahead of the funeral procession, which drew thousands of people who fired their rifles in the air as a sign of mourning.
Abdul-Wahid and his bodyguard were gunned down Sunday by a Lebanese soldier, apparently after the men failed to stop at an army checkpoint.
The exact circumstances of the shooting remain murky, however, fueling already boiling tensions in Lebanon between pro- and anti-Syrian factions. The countries share a complex web of political and sectarian ties and rivalries, which can easily turn violent.
"They killed him because he supported the Syrian revolution, he was very active," said Abdel-Aziz Dandal, a 45-year-old resident of Bireh.
He said the cleric had been recently elected head of the local council and was expected to become the head of a regional council for about a dozen villages. Many residents said he had made support for Syrian refugees and the uprising central to his work.
The killings unleashed the most serious outbreak of violence in the Lebanese capital since the Syrian uprising began 15 months ago, as Lebanese Sunni groups supporting and opposing the Damascus regime fired rocket-propelled grenades and machine guns through the night.
At least two people were killed and 15 were wounded. Last week, several days of clashes sparked by the Syrian crisis killed at least eight people and wounded dozens in the northern city of Tripoli.
Police commander Maj. Gen. Ashraf Rifi toured Beirut's Tariq Jadidah neighborhood, scene of the overnight clashes, and said "things will be getting better." He said police and armed forces sent patrols into Tariq Jadidah to "reassure the people."
The streets were calmer by Monday morning but some shops remained closed and many parents kept children home from school.
The battles pitted supporters of the anti-Syrian Future Movement, the party of the former Prime Minister Saad Hariri, against those of the pro-Syrian Arab Movement Party. The U.S. expressed concern over the security situation in Lebanon.
"We welcome the commitment of the Lebanese government and the Lebanese Armed Forces to conduct a swift and transparent investigation of the shooting incident, and we call on all parties to exercise restraint and respect for Lebanon's security and stability," U.S. State Department spokesman Mark Toner said.
The Shiite militant group Hezbollah, which is the most powerful armed force in Lebanon, did not appear to have any direct role in the fighting. Hezbollah is a strong ally of Syria.
There is an array of die-hard pro-Syrian parties and politicians in Lebanon, as well as support for the regime on the street level.
But there is an equally deep hatred of Assad among other Lebanese who fear Damascus is still calling the shots here. The two sides are the legacy of Syria's virtual rule over Lebanon from 1976 to 2005, when Syrian troops were driven out.
During the time of blatant Syrian dominance, Lebanese leaders used to travel frequently to Damascus to get marching orders.
Tensions between the two sides are high enough. But Syria opponents worry the regime may intentionally cause trouble.
The Lebanese army has promised to investigate the cleric's killing. But both sides of the conflict are polarized. Regime opponents say Syria is trying to stir up trouble in Lebanon to deflect attention from the troubles at home, and to show that Assad is capable of destabilizing the region if threatened.
But the pro-Syrian factions say it's not in their interest to see chaos in northern Lebanon, where rebels are trying to smuggle fighters and weapons into Syria. Assad has long said the revolt against him is the result of a foreign conspiracy.
Amid fears the situation might deteriorate, four Gulf countries - Kuwait, Qatar, Bahrain and the United Arab Emirates - have warned their citizens against travel to Lebanon. The warnings are a blow to Lebanon's summer holiday season, a pillar of the economy.
The Supreme Islamic Legislative Council, the top Sunni religious body in Lebanon, described Abdul-Wahid's killing as an "assassination crime par excellence" and urged self restraint. It also wants to refer the killing to the Judicial Council, which is charged with cases seen as destabilizing national security.
The Beirut clashes were among the most serious battles in the capital since May 2008, when Hezbollah gunmen swept through Sunni neighborhoods after the pro-Western government tried to dismantle the group's telecommunications network. At the time, more than 80 people were killed in the violence, pushing Lebanon to the brink of civil war.
Inside Syria on Monday, activists said regime forces killed dozens of people the previous day in a raid on the central town of Soran in Hama province.
One activist group, the Britain-based Syrian Observatory for Human Rights, put Sunday's 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.
As the violence intensifies, there are growing fears that al-Qaida or other extremists could be joining the fray. In a statement posted on a militant website late Sunday, a group calling itself the Al-Nusra Front claimed responsibility for a suicide car bombing Saturday in the eastern city of Deir el-Zour that killed nine people.
Little is known about the group, although Western intelligence officials say it could be a front for a branch of al-Qaida militants from Iraq operating in Syria. The group claimed responsibility for several other suicide attacks in Syria.
The U.N. has an observer mission in the country with about 270 unarmed monitors, but it has not stopped the violence.
U.N. peacekeeping chief Herve Ladsous warned Monday that terrorists are trying to exploit the crisis between the government and the opposition.
"A third party, terrorist groups, are trying to gain advantage for themselves," he said during a press conference in Damascus.
On Monday, residents in Lebanon said their problems will persist as long as there is unrest in Syria. "The Syrian regime has supporters here and they are inciting," said Dandal, the Bireh resident. "The problems will continue as long as there are problems in Syria."