[1] Early in the morning of April 13, 1975, outside the Church of Notre Dame de la Delivrance at the predominantly Maronite inhabited district of Ain el-Rammaneh in East Beirut, an altercation occurred between half a dozen armed Palestine Liberation Organization (PLO) guerrillas (Arabic: Fedaiyyin) on a passing vehicle performing the customary wavering and firing their automatic rifles into the air (Arabic: Baroud)[2] and a squad of uniformed militiamen belonging to the Phalangist Party's Kataeb Regulatory Forces (KRF) militia,[3] who were diverting the traffic at the front of the newly consecrated church where a family baptism was taking place.

[4][5][6] Among the dead in the drive-by shooting were Joseph Abu Assi, an off-duty Phalange militant and father of the baptised child, and three bodyguards – Antoine Husseini, Dib Assaf and Selman Ibrahim Abou, shot while attempting to return fire on the assailants[7][8][9][10][11] – of the personal entourage of the Maronite za'im (political boss) Pierre Gemayel, the powerful leader of the right-wing Phalangist Party, who escaped unscathed.

In the commotion that followed, armed Phalangist KRF and NLP Tigers militiamen took the streets, and began to set up roadblocks at Ain el-Rammaneh and other Christian-populated eastern districts of the Lebanese Capital, stopping vehicles and checking identities,[12] while in the mainly Muslim western sectors the Palestinian factions did likewise.

It sparked heavy fighting throughout the country between Kataeb Regulatory Forces militiamen and the Palestinian Fedaiyyin and their leftist allies of the Lebanese National Movement (LNM) alliance, resulting in over 300 dead in just three days.

The chain of events that led to the Ain el-Rammaneh church shooting and the subsequent "Bus massacre" (or "Black Sunday") of April 1975 have been the subject of intense speculation and heated debate in Lebanon since the end of the Civil War in 1990.

Although most PLO accounts refute this version of the events by describing the bus passengers as civilian families' victims of an unprovoked attack and not fully armed guerrillas, Abd al-Rahim Ahmad of the ALF did confirm years later that some of them were off-duty members of that faction.

Indeed, critics pointed to the all-too-obvious presence of civilian automobiles plastered with propaganda of that PLO faction and the tactic employed (a drive-by shooting), which did not fit well into the methods commonly used by the Palestinian guerrilla movements at the time.