In a bloody sequel to renewed violence and rising tension in Beirut, the explosion of a booby-trapped car today killed at least 12 persons and wounded 118 others in a densely populated low-income Christian suburb.

A beige Mercedes-Benz packed with hundreds of pounds of explosives, mortar shells and nails exploded at midmorning in the neighborhood of Sad Al Bawshrieh, just north of the Christian-controlled eastern sector of the Lebanese capital. The bomb followed a new round of fighting across the demarcation between Christian east Beirut and Moslem west Beirut, known as the Green Line, and the mountain ridge to the southwest.

It was unclear whether today's car bomb was linked to the rising tension or to an internal Christian struggle for influence between Christian militias and the more moderate Phalangist Party followers of President Amin Gemayel.

The blast brought down the facade of one apartment block, damaged four other buildings, burned scores of cars parked in the narrow street and sent glass and debris flying over an area several yards wide.

Red Cross workers, civil defense teams and firefighters rushed to the scene to rescue residents trapped on upper floors and to transport casualties to nearby hospitals. Charred bodies and limbs were heaped on stretchers as firefighters struggled to extinguish the blaze caused by the explosion.

Hospitals located in east Beirut appealed for blood donations and called on parents to claim children hospitalized for injuries. Hysterical mothers collapsed in the arms of rescue workers at the sight of babies being carried to ambulances. State-run Lebanese television showed gruesome footage of bloodied extremities at the scene, where Christian militiamen kept away curious onlookers.

The carnage in Sad Al Bawshrieh followed an upsurge in artillery, mortar and rocket battles in the city and the hills overlooking it. This latest convulsion of sectarian hostilities has left 30 dead and 140 wounded.

Moslem and Christian opposition leaders met in the central Lebanese town of Shtawrah last week to draw up a list of priorities in envisioned political reforms aimed at ending the domination of Lebanon's Christian Maronite minority. Rather than spur dialogue, this conference, coupled with changing alliances within the Christian camp, seems to have fueled Moslem-Christian tension.

Ex-president Suleiman Franjieh's unexpected reconciliation with the commander of the Lebanese Forces militia, Elie Hobeika, raised suspicions among Moslem leaders that the new Christian alignment may stand in the way of demands for a more balanced power structure.

Camille Chamoun, another former Christian Maronite president, attributed the latest spate of vio-lence to the formation of the National Unity Front in Shtawrah. The Syrian-sponsored front vowed to eliminate "political sectarianism," a reference to Lebanon's four-decade-old system of power distribution according to religious sect.

He said the latest deterioration in security conditions was "closely linked to the Shtawrah conference," the purpose of which is to "Islamicize Lebanon."

Justice Minister Nabih Berri, leader of the Shiite Moslem militia, yesterday swore to "punish" Christian units of the Lebanese Army, which he accused of pounding his militia with tank fire in a predawn attack Tuesday across the midcity Green Line. Security sources said two Lebanese soldiers were killed overnight along the Suq Al Gharb axis, a two-year-old front line between the Army and Druze militias.

A toughly worded Lebanese Army communique issued today after a long silence warned those who were behind the latest escalation in Beirut and the Suq Al Gharb mountain ridge to "be aware of their responsibilities to avoid worse things to come, which may exceed all limits and shatter [established] lines."

The rising tone of bellicosity between Christian and Moslem politicians does not bode well for declared Syrian aims to pacify Beirut. Berri's Amal militia received 50 Soviet-made T54 tanks from Syria two weeks ago.