Attacks against mosques and churches and the assassination of two clergymen last week have triggered fears here that major sectarian violence may ravage Lebanon again and have challenged residents' short-fused tolerance of other religious communities.

Today, a powerful car bomb blew up in front of a mosque under construction, injuring four persons. The mosque is just 200 yards away from the home of the Sunni Moslem grand mufti, Sheik Hassan Khalid.

Last week, a senior Sunni Moslem clergyman, Sheik Ahmed Assaf, was slain in mainly Moslem West Beirut and a bomb was hurled against a mosque near the home of the Sunni former premier Saeb Salam. Some observers here saw these attacks as part of a plot to arouse sectarian sensitivities in Lebanon. These suspicions were reinforced a few days later by the killing of a Maronite Christian priest, Fr. Philip Abu Suleiman, in the mainly Druze resort of Aley in the Mount Lebanon area.

The attacks, particularly the slaying of Assaf, led to a protest strike called by Moslem leaders and supported by many influential members of the Christian community in an attempt to defuse the tension.

Last Sunday, three persons were killed and five others seriously injured inside a Maronite Christian church in the northern port city of Tripoli after a lone Moslem gunman forced his way inside, shooting into the air and demanding the priest's attention.

The assailant, later identified as Omar Louz Badawiyeh, 33, charged into the church brandishing a gun and a hand grenade. He was seeking the priest's help to obtain the release of a brother reportedly kidnaped by Phalange militiamen in Christian-controlled East Beirut three months ago.

When security guards exchanged fire with Badawiyeh, he tossed the grenade, instantly killing himself and a woman worshiper. Six others were badly wounded. One of them, a young girl, died of her wounds later.

Whether by accident or design, the nature and brief intervals between these incidents unleashed an atmosphere of fear.

The killing of Assaf, a well-respected Moslem cleric and intellectual known for his moderation in political stands and efforts to keep up contacts with all Lebanese communities, led Shiite, and Sunni Moslem and Druze religious chiefs as well as traditionalist Moslem leaders to call for a protest strike.

Surprisingly, the Christian Lebanese Front, led by former president Camille Chamoun and Phalangist leader Pierre Gemayel, responded to the call and enforced the strictest strike ever in Christian areas. This unprecedented show of solidarity was appreciated in Moslem quarters.

Walid Jumblatt, Druze leader and head of the National Movement, an alliance of progressive Moslem and leftist groups operating here, had his movement observe the nationwide strike but expressed skepticism that the joint action would reduce factional tensions.