Rebels in northern Syria kidnapped 13 Lebanese Shiite Muslims in Aleppo province on Tuesday as they were headed back home by bus from a pilgrimage in Iran, Lebanese authorities said.
Reports of the kidnappings prompted families of those abducted, along with thousands of supporters, to gather in Beirut's mainly Shiite southern suburbs to demand their release.
The protesters closed down several roads, including the old airport road, with burning tyres and garbage bins.
There were also reports of roads being closed in the eastern Bekaa valley over the kidnappings
Hezbollah's Al-Manar television said the head of the militant movement, Hassan Nasrallah, would speak about the kidnappings in a televised address at 7:30 pm (1630 GMT).
Prime Minister Najib Mikati's office said he was making the necessary contacts to ensure the release of the men.
"Prime Minister Mikati has urged families of the kidnapped to remain calm and assured them he was following the issue closely to insure the safety of those kidnapped and their quick release," a statement said.
One man who refused to give his name said his two bothers-in-law were among those abducted.
"They were heading back to Beirut on board a bus after visiting religious sites in Iran," said said man, who refused to give his name.
"The women who were with them were allowed to go free."
The brother of Abbas Shaayb, who organised the pilgrimage, said the women were staying in a hotel in Aleppo.
"Let's see what the friends of the Free Syrian Army in Lebanon are going to do now," said the man, referring to the Sunni-led opposition in Lebanon that has backed the 14-month uprising in neighbouring Syria.
The reported kidnappings were sure to further inflame sectarian tension in Lebanon over the revolt in neighbouring Syria.
Clashes between the pro- and anti-Damascus camps in the country have left some 12 people dead in the past 10 days.
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