Rebels in northern Syria kidnapped a group of Lebanese Shiite Muslims in Aleppo province on Tuesday as they were headed 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. The roads were reopened later in the evening.
Lebanon's state news agency put the number of those abducted at 13 while Syrian media said an "armed terrorist gang" had kidnapped 11 Lebanese and their Syrian driver.
Hezbollah leader Hassan Nasrallah urged restraint and said his Shiite militant party was doing its utmost to ensure the safe release of the men.
"I call on everyone to show restraint," Nasrallah said in an address carried on the party's Al-Manar television station. "It is not acceptable for anyone to block roads or carry out violent acts."
Nasrallah said contacts were underway with Syrian authorities and other countries in the region for a quick resolution.
"We will work day and night until those beloved are back with us and hopefully ... by cooperating together we will reach a happy ending," he said.
Nasrallah said Hezbollah was also in contact with Prime Minister Najib Mikati, whose government is dominated by the powerful militant group allied with the regime in Syria.
"The Lebanese state and government have a responsibility to work toward the release of those kidnapped," he said.
Nasrallah urged his followers not to carry out revenge attacks against Syrians in Lebanon.
Syrian state media said the kidnapping took place near the town of Aazaz, which sits along the border with Turkey. State news agency SANA said the men were part of a group of 53 pilgrims on two buses.
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Activists in Aleppo reached by AFP via Skype said Aazaz was undergoing fierce shelling Tuesday evening by regime forces who stormed the town.
The brother of one of those kidnapped said the rebel Free Syrian Army seeking to topple the regime of Bashar al-Assad had vowed to release the men in exchange for FSA members detained by Syrian authorities.
Mikati's office said he was making the necessary contacts to ensure the release of the Lebanese abducted.
"Prime Minister Mikati has urged families of the kidnapped to remain calm and assured them he was following the issue closely to ensure 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 ... after visiting religious sites in Iran," said the man. "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 pending their return to Lebanon.
"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 likely to further inflame sectarian tensions in Lebanon over the Syrian crisis.
Clashes between the pro- and anti-Assad camps in the country have left some 12 people dead in the past 10 days.
Nasrallah said it was necessary for all Lebanese to remain calm.
"The atmosphere is tense because of the events of recent days," he said. "Everyone is urged not to make matters worse."
Ex-premier Saad Hariri, who heads the opposition in Lebanon, denounced the kidnapping and called for the men's immediate release.
"We condemn the kidnapping of our Lebanese brothers in Syria, regardless of the party behind the kidnapping, and we call for their immediate release," he said in a statement.