The Palestinian Fatah movement clashed with members of unknown Islamic groups, using machine guns and rocket-propelled grenades.

Civil defense teams worked alongside the Lebanese army to put out fires caused by the fighting.

The clashes erupted on Saturday despite a ceasefire agreement on Friday evening that sought to put an end to fighting in the camp between the rival groups on Thursday and Friday.

The two days of fighting left nearly two dozen dead in the densely populated, walled-off camp.

Over the years, Islamist militants have established a presence in Ain Al-Helweh, the largest of Lebanon's 12 Palestinian refugee camps.

Its estimated 54,000 residents have been joined in recent years by some 6,000 Palestinians who sought refuge from Syria's civil war, all crammed in an area covering no more than two square kilometres (0.8 square miles).

The camp, with its narrow alleyways, is surrounded by a wall erected by the Lebanese army, whose forces control all four entrances 

A 1969 agreement between Beirut and the Palestine Liberation Organization (PLO) stipulated that security in Ain Al-Helweh and the other refugee camps was to be handled by Palestinian factions rather than Lebanese forces.

The Lebanese government annulled the deal in 1987, but the army still does not enter Palestinian camps by long-standing convention.

Fatah, which controls the PLO and the Palestinian Authority, remains the most influential actor in Ain Al-Helweh, though rival factions like Hamas and Islamic Jihad threaten its hegemony.

Parts of the camp have also become bastions of Islamist cells of Lebanese and Syrian nationals, which include some fugitives.

One of the better known of them is Lebanese-Palestinian singer Fadel Shaker, who had been sentenced to 15 years in prison for supporting local Sunni extremist groups.

According to UNRWA, the United Nations' agency for Palestinian refugees, more than 80 percent of Palestinian refugees in Lebanon live in poverty.