Lebanon's army arrested on Wednesday 15 people in the eastern town of Arsal, where many Syrian civilians and rebels took fled to since the Syrian army’s takeover of Yabrud last week.
Mostly Sunni Arsal has been sealed off by residents from the mainly Shiite neighboring town of Labweh, where one person was killed last week by rocket fire.
On Wednesday, more rocket fire launched from across the border hit two areas of eastern Lebanon, causing no casualties, the army said.
The closure of the main road out of Arsal prompted fury among Sunnis in other parts of Lebanon, and many took to the streets on Tuesday night to burn tyres in protest.
One person was shot dead at a demonstration in the capital Beirut.
While lawmakers called for an investigation into his killing, Sunni protesters briefly blocked roads in several areas of the capital for a second consecutive day, according to AFP.
Amid the tensions, President Michel Sleiman met Prime Minister Tammam Salam and senior security officials, the official National News Agency (NNA) reported.
People in Labweh blame residents of Arsal for launching rockets at their town, although the fire was reported to have originated across the border in Syria.
Tensions between residents of the two towns have increased since the fall on Sunday of the Syrian rebel bastion of Yabrud, just across the frontier.
Its capture prompted an exodus of civilians and rebels, with many taking refuge in or around Arsal, where residents are sympathetic towards the Syrian uprising.
The army arrived early Wednesday and reopened the road, Arsal municipal council member Bakr Houjairi told AFP.
"We are happy to see the army here and opening the road up," he said.
The conflict in Syria has exacerbated existing sectarian tensions in Lebanon, particularly between Sunnis and Shiites.
Many Sunni Lebanese back the Sunni-dominated Syrian uprising, while Shiites support Lebanon's powerful Shiite Hezbollah movement, which is allied with Syria's regime and fighting alongside its forces.
Labweh residents welcomed the deployment of the army, according to Hezbollah MP Hussein al-Hajj Hassan, who told parliament that they "have one demand, and that is for the army to deploy in Arsal".
The border region has become particularly fragile, with Arsal hosting some 51,000 civilian refugees and facing accusations that it has allowed rebels to establish bases in the area.
Four rockets fired from across the border hit the Labweh and Al-Nabi Othman areas in the Hezbollah-dominated Bekaa Valley in eastern Lebanon, the military said on Wednesday, adding that one man was wounded.
Lebanon's northern city of Tripoli has also seen frequent armed clashes between Alawite supporters of Assad and Sunni backers of Syria's rebels.
On Wednesday, a man died of gunshot wounds, seven days into clashes in Tripoli, taking the total death toll from the latest round of fighting there to 13, a security source said.
Hundreds of people have died in Syria-related violence in Lebanon since the anti-Assad revolt erupted three years ago.
[With AFP]SHOW MORE