The camp is where many former residents from the Nahr al-Bared refugee camp had taken emergency shelter during fighting between Fatah al-Islam and the Lebanese army, which had lasted for 15 weeks. Ismail's wife remains in Beddawi.
Abu Ali Fares, a spokesman for a Palestinian faction in the Beddawi camp, said: "Nasser Ismail was captured by a [Palestinian] security force ... the force raided the house of a relative of Nasser Ismail and found him hiding in the attic with another person."
"He was taken aboard a Red Crescent ambulance during the night of Sunday to Monday. He was handed over to the [Lebanese] army intelligence services."
Death toll rises
Khalil Dib, an official of Fatah al-Intifada, a Palestinian faction, has said that Ismail, while in the custody of the Palestinian forces, had told him that he had been in Nahr al-Bared until Saturday before heading to Beddawi.
Since government forces defeated Fatah al-Islam fighters on September 2, the Lebanese army has been searching the area for militants, including Shaker al-Absi, the leader of Fatah al-Islam.
Dib also said that according to Ismail, "Shaker al-Absi left Nahr al-Bared one month before the end of the battle".
Abu Salim Taha, Fatah al-Islam's spokesman, was captured near Beddawi camp last month.
Abu Hureira, the deputy commander, was killed in a shootout with security forces in August after he fled the camp.
More than 400 people have died in the fighting between the Lebanese army and Fatah al-Islam, including at least 222 fighters.
A Lebanese soldier died last Friday, raising the army death toll to 168.