Thousands of Lebanese civilians and Syrian refugees have crammed into cars and trucks and fled as Lebanese artillery pounded a border town that had been over-run by militants from neighbouring Syria.
The civilian exodus came during a relative lull in fighting and just a few hours later the bombardment around the town of Arsal had reached an intensity of three shells every minute.
The fighting is the most serious spill-over of violence from Syria's civil war into Lebanon, compounding fears that tiny Lebanon is fast becoming a new front in its neighbour's conflict, now in its third year.
The government has rushed reinforcements to the scene, including dozens of armoured carriers and tanks.
The three days of clashes in Arsal, a predominantly Sunni town surrounded by Shiite villages, could worsen already bubbling sectarian tensions in Lebanon. The Syrian government, which is battling a largely Sunni insurgency, has the support of Lebanon's main Shiite militia, Hezbollah.
The town of 40,000, whose population has almost tripled because of the presence of Syrian refugees and rebels, is wedged between Syrian government-controlled territory and Lebanese Shiite villages sympathetic to Hezbollah.
A senior Lebanese security official said 17 soldiers have been killed in three days of fighting, including two lieutenant colonels, and 13 others were missing.
Lebanon's state-run news agency said the rebels were looting homes and shops in Arsal and a resident on the outskirts told the Associated Press that the militants were committing "atrocities" and shooting at people attempting to flee.
"The rebels feel protected by the civilians there," he said, confirming there was widespread looting with rebels taking over civilian homes to use as military posts.
Among those fleeing was Aziza Rayed, in her 60s, who said her family was going to the nearby border town of Qaa.
"We are leaving to take these children to a safer place," she said, her children and grandchildren in the back of a pick-up truck.
Syrian refugees who had earlier fled the war at home for Arsal's safety found themselves on the road again.
Fatmeh Meshref, from the Syrian central city of Homs, said she and her husband and five children were terrified.
"Our children were screaming and we had no place to hide," she said.