There have been scenes of chaos as rebels from Syria raided a border town in Lebanon, killing members of the security forces and abducting others.
Fighters said to be from the Nusra Front briefly seized a police station in Arsal, which is home to thousands of refugees from the war in Syria.
On Sunday morning, the fighting was continuing in the mainly Sunni town.
The area has been the scene of regular tension between the Lebanese army and Syrian militias.
Previous border incursions never went so deeply into Lebanese territory.
One report says four Lebanese nationals were killed, including two soldiers, and several others were seized, as rebels roamed the streets while Lebanese helicopters hovered overhead.
The Lebanese army says eight of its soldiers have been killed in clashes, other reports say.
Lebanese Prime Minster Tammam Salam described the assault as a "flagrant attack on the Lebanese state".
A Nusra Front spokesman told Reuters news agency they were demanding the release of a leader, Emad Jumaa, after he was arrested at a checkpoint near the town, which is predominantly Sunni Muslim.
Lebanon's army, which rushed tanks to Arsal, said in a statement it "would not allow any party to transfer the battle from Syria to its land".
Meanwhile, Hezbollah fighters from Lebanon's Shia Muslim community are said to have taken part in an ambush on rebels inside Syria on the side of President Bashar al-Assad.
At least 50 rebels, including members of the Nusra Front, were killed in the attack in the Qalamoun area near the Lebanese border on Saturday, activists and security sources said.
The Nusra Front, al-Qaeda's Syrian affiliate, has been fighting along with other rebel groups against the Syrian government but has also clashed with fellow jihadist group Islamic State (IS), which is seeking to carve out a new Islamist state straddling Syria and Iraq.
Lebanon now hosts more than a million Syrian refugees, meaning a quarter of its total population is Syrian.