Four people were wounded when two rockets exploded in the Shiite-majority Hezbollah heartland of south Beirut, a Lebanese security source said.
"Two Grad rockets hit the southern suburbs of Beirut. One rocket struck a car showroom where four people were wounded and vehicles were damaged," the source told AFP on Sunday.
It was the first time the Lebanese capital's southern suburbs have been targeted during the two-year-old conflict in neighbouring Syria, where Hezbollah has thrown its military might into the regime's fight against rebels seeking to topple President Bashar al-Assad.
Sunday's blasts came just hours after Hezbollah leader Hassan Nasrallah vowed "victory" in Syria.
"I say to all the honourable people, to the mujahedeen, to the heroes: I have always promised you a victory and now I pledge to you a new one" in Syria, he said at a ceremony marking the 13th anniversary of Israel's military withdrawal from Lebanon.
Nasrallah said Hezbollah would always stand by its ally Assad and his regime, stressing that its own interests were at stake.
The fighting in Lebanon has already spilled over into the country's second city, the northern port of Tripoli, where 30 people have been killed in a week of clashes between pro-Assad Alawites and pro-rebel Sunni Muslims.