Two car bombs hit Moslem neighborhoods in west Beirut today, apparently in retaliation for car-bomb attacks against two Christian areas last week. Police said 29 people were killed and at least 70 injured in today's explosions.

The bombs were detonated shortly after noon on crowded streets outside a small cafe and near a mosque. Powerful car bombs last Wednesday and Saturday killed 68 people and injured 250 in Christian east Beirut and a suburb.

The apparent exchange of car-bomb attacks raised tensions in this war-torn city. The state-run Beirut television declared that "Lebanon is turning into an inferno," and the Moslem-run Voice of the Nation warned that "we have a car-bomb war on our hands now."

While Moslem militiamen at the scene of today's bombings called for revenge, Moslem and Druze leaders continued to blame Israel for the attacks and avoided the retaliatory threats that some Christian leaders have made.

President Amin Gemayel, a Christian, said that "these incidents are targeting the Lebanese citizen regardless of his identity or religion," and he charged that the main objective was to "disrupt the Syrian role and to drag Lebanon back into an atmosphere of killing and violence." Syria has been attempting to persuade rival Lebanese factions to resolve their differences.

Labor Minister Selim Hoss, a Moslem, said he refused to believe that today's attacks were a direct response to last week's car bombs. Hoss charged that the "hands of crime reaching into east and west Beirut are one and the same" -- an apparent reference to Israel.

Despite the Lebanese government's demonstrated inability to maintain law and order, Prime Minister Rashid Karami announced a meeting Thursday of the country's "national unity" Cabinet, the first for the badly divided body since April. But two key figures -- Druze leader Walid Jumblatt and Nabih Berri, head of the Shiite Moslem Amal militia -- were not expected to attend.

An anonymous caller claimed responsibility for today's bombings on behalf of the previously unknown Black Brigades and told a Beirut-based foreign news agency that any attempts to wipe out the Christians of Lebanon would be countered.

"We shall confront the war of annihilation being waged against the Christian people with a counterwar of annihilation," the caller said. "If Lebanon will not be safe for Christians, it will not be safe for anyone else."

The authenticity of such calls is impossible to prove, but the tone of the statement captured a rising mood of intolerance among Moslems and Christians. There have been no claims of responsibility for last week's bombs in the Christian areas.

The first of today's bombs was detonated in front of a small cafe in the Zarif area, setting dozens of automobiles ablaze and demolishing store fronts. Within minutes, another car packed with explosives blew up a few yards from a mosque in Ghobairi, rocking that populous quarter and killing four persons.

At the site of the first bomb, which killed 25 persons, enraged Moslem militiamen used their guns to smash windshields in search of more explosives. The bomb turned cars into piles of burning metal and sent glass flying 100 yards. A 10-year-old girl cried that her two sisters were killed in the blast.

As tensions mounted here today, the three crossings linking Moslem and Christian areas were shut and shelling against residential areas resumed.

Reduced to a ghetto existence by this latest spasm of sectarian violence, Beirut's two main religious communities are now hostage to the daily nightmare of car bombings and indiscriminate shelling. The apparent absence of any solution and the expressed unwillingness of leaders for political compromise and dialogue have drained most residents of hopes for improvement.

An Amal militiamen vowed reprisals against Christians. "Our reply will be much more violent. If they want to play the game of death, we know how to play too," he yelled at a reporter.

Berri, however, scoffed at a warning over the weekend by former president Camille Chamoun, a Christian, that the perpetrators of the bombings in Christian areas must be aware that "they will get two blows for each one delivered."

"As to the threats for retaliation, the message arrived today in the suburbs and west Beirut," Berri said. "We shall not respond in kind to these acts, because it is not our style." But he warned that any shelling of Moslem areas "will push us toward a military showdown."

Low-key statements by Moslem leaders after last week's bombings did little to temper the rage of the Christian community, clinging to its last symbols of power and illusions of safety. Christian families have been moving from the predominantly Moslem half of Beirut in search of tranquility in the Christian heartland -- now confined to east Beirut, a stretch of coast to the north and mountain villages to the northeast.

Gemayel today received letters from Syrian President Hafez Assad and his vice president, Abdul Halim Khaddam, but there was no indication of their content. Syria stepped in as Lebanon's main power broker last year after the Beirut government, under Syrian pressure, canceled a U.S.-sponsored Lebanese-Israeli troop withdrawal agreement.

Despite its apparent efforts to get Lebanon's warring factions to negotiate a formula allowing for a more balanced power structure between Moslems and Christians, Syria has not succeeded in drawing its main local allies away from the gun and to the bargaining table.

Meanwhile, in southern Lebanon, Stephan Jacquemet, a Swiss national who represents the International Committee of the Red Cross in Sidon, was kidnaped by unidentified gunmen near Adloun, between Sidon and Tyre.