A car bomb exploded outside a supermarket crammed with weekend shoppers in a Christian suburb north of Beirut today, killing at least 50 persons and wounding at least 100, many of them seriously.

The explosion, apparently detonated by remote control, was the second in four days against Christian civilians here. It blew several bodies across a coastal highway and into the Mediterranean, 300 yards away, and left others charred and mangled inside the supermarket and in cars parked outside. Smoke killed several residents of apartments in the upper floors of the six-story building.

There was no immediate claim of responsibility for the bombing. In the past week, fighting and shelling between Christian and Arab forces had killed at least 48 persons and wounded 250 before today's attack. Last Wednesday, another car bomb in a heavily populated east Beirut neighborhood killed 13 persons and injured 120.

The executive committee of the Lebanese Forces, the largest Christian militia, threatened revenge today, warning those responsible that "we will retaliate, and our response will be of the same dimensions as their crime."

The Christians did not directly accuse the Moslem militias, but it was clear that was whom they blamed.

"Indiscriminate shelling of our areas and bombing attacks against our civilians have reached the limit," the Lebanese Forces' statement said. "Those criminals have trampled on all norms and rejected peace, asking for a dirty war."

Prime Minister Rashid Karami, a Moslem, called the bombers "wild beasts," but acknowledged his and the government's helplessness in the face of the civil warfare.

"What can I say, and what can I do?" he asked. He said, without elaborating, that there were "major and forceful powers planning and executing" such acts.

Justice Minister Nabih Berri, leader of the Shiite Moslem Amal militia, who only days ago warned that he would punish Christian units of the Army that he accused of launching attacks against his militia, charged that Israel was behind today's explosion.

Some political analysts said rivalries between Christian factions could not be ruled out in looking for the perpetrators, but they said it was unlikely that any Christian militias would choose a purely civilian target to settle scores.

Bomb experts estimated that today's blast was caused by 500 pounds of explosive.

Flames quickly engulfed the entrance of the supermarket in Antelias, about three miles up the coast from Beirut, and spread to upper floors. The pressure of the blast knocked bystanders unconscious and left the remains of the boobytrapped car dangling from overhead power lines. Survivors stumbled out of the billowing smoke with blistered arms and legs, blood streaming down their faces.

Civil Defense chief Elie Honein, supervising the rescue operations, said it was impossible to give an accurate casualty count.

"All we are getting out are charred bodies," he said. "People were crowded in there buying their provisions for the weekend. The car bomb came, and within seconds, turned them to ashes. Very few are coming out alive."

Supermarket owner Nabil Melki, who had been at home at the time of the explosion, said he had 50 persons working inside and did not know how many had survived.

"My brothers, brothers-in-law and nephew were inside; I don't know where they are now," Melki said distractedly as he watched rescue crews rush in and out of his smoldering supermarket.

Militiamen from the rival Lebanese Forces and the Phalangist Party milled around at the explosion site, their rifles pointed skyward, and looked warily at one another.

The last two car bombings have occurred in areas under the control of Christians loyal to President Amin Gemayel and the Phalangist Party. The Lebanese Forces gained the upper hand in Christian politics after a power move last March by one of its militant commanders.

The political climate has worsened despite apparent efforts to push ahead with proposed reforms. Leftist and Moslem leaders stressed last week that a blueprint for changes drawn up in a meeting in Shtawrah, in central Lebanon, was not negotiable but the only solution left for Lebanon.

That conference, held under the sponsorship of Syria, now Lebanon's main power broker, called for the elimination of a system that favors the Christian minority in senior government posts and apportions power according to outdated ratios of religious affiliations.