A pair of rockets have slammed into a car dealership and a residential building in strongholds of Lebanon's Hezbollah militia in southern Beirut, wounding four people.
The attack has raised fears that Syria's civil war is increasingly spreading into Lebanon.
Lebanon's sectarian divide mirrors that of Syria, and Lebanese armed factions have taken sides in their neighbor's civil war.
There was no claim of responsibility for Sunday's attack.
However, a Syrian rebel commander threatened earlier this week to strike against Hezbollah strongholds in retaliation for the militia's military support for Syrian President Bashar al-Assad.
Hezbollah is a Shia Muslim group, while most of the rebels are Sunnis.
Street fighting between rival Lebanese groups has been relatively common since the end of the country's 15-year civil war in 1990, but rocket or artillery attacks on Beirut neighborhoods are rare.
The rockets struck hours after Hezbollah leader Hassan Nasrallah said his group would stay in the Syrian war "to the end of the road" and bring victory to its ally President Bashar al-Assad.
Mr Nasrallah, head of the militant Shia Muslim group, said in a televised speech that Syria and Lebanon were facing a threat from radical Sunni Islamists, which he argued was a plot devised by the United States and its allies to serve Israel's interests in the region.
"We will continue to the end of the road, we accept this responsibility and will accept all sacrifices and expected consequences of this position," he said in the televised footage, which showed thousands of cheering people watching him on big screens in the Lebanese town of Mashgara.
Mr Assad comes from the minority Alawite sect, an offshoot of Shia Islam.
He has been bankrolled by regional Shia power Iran and now increasingly by Hezbollah, Tehran's Lebanese proxy which was founded as a resistance movement to Israel.
Violence from the Syrian conflict, which began as a peaceful protest movement but descended into civil war, has increasingly spilled over into Lebanon, particularly in the northern city of Tripoli.