Lebanese Prime Minister Rashid Karami said today that he would boycott sessions of his national unity Cabinet until decisions were carried out to put an end to fighting in Sidon.

Karami declared his intention to suspend active participation in his own government after the Cabinet listened to a report by the Army command on delays in sending more troops and materiel to Lebanese soldiers in the embattled port city of Sidon, south of Beirut. The stand of the Sunni Moslem prime minister signaled general Moslem discontent with the inability of the Army to halt communal hostilities that broke out three weeks ago between Christian militiamen entrenched on Sidon's eastern fringes.

Education Minister Selim Hoss, also a Sunni Moslem, declared support for Karami. Two other Moslem ministers, Shiite Justice Minister Nabih Berri and Druze Tourism Minister Walid Jumblatt, have been staying away from Cabinet sessions for several weeks.

Two battalions and 28 U.S.-supplied armored personnel carriers have been in place near Beirut airport to head south for the past couple of weeks. Fears that Christian militiamen would confiscate the new equipment en route to Sidon or that Army elements would disobey orders in fighting their coreligionists have stalled the plan.

An estimated 40,000 people have been displaced from their homes in townships east of Sidon, and Palestinian refugees have fled from the camps of Ain Helweh and Miyumiye to the center of the city.

Karami noted that he will not resign yet but he was unequivocal in stating that he could not reverse his position unless measures concerning Sidon are implemented.

"We are still facing snags delaying implementation of steps to save Sidon. Since the remedies agreed on by this Cabinet have not come to fruition, I will not attend Cabinet meetings until all these problems are resolved and decisions are implemented," he told reporters.

In the past, Karami, who has served as Lebanon's prime minister 10 times, has chosen the boycott option when he was opposed to the use of the national Army. He did so for 10 months during the 1975-76 civil war because he objected to the deployment of Army troops against Palestinian guerrillas and Moslem militiamen. In 1969, he used the same method to force President Charles Helou to sign an agreement that regulated an armed Palestinian presence in Lebanon.

Karami has been embarrassed by his government's failure to come to the rescue of Sidon, a largely Sunni town. The Lebanese forces and Israeli-backed militiamen of the South Lebanon Army have been shelling Sidon and its environs and have kept the residents who remained there pinned down by sniping.

Moslem leaders and moderate Christian figures have accused Israel of fomenting trouble in the south with an eye toward establishing a front line east of Sidon as a first step toward demarcation of a swath of Christian-controlled territory running south to the border.

Forty-six Christian religious leaders and politicians met yesterday at the seat of the Maronite patriarch, Cardinal Antonios Boutros Khreish, and issued a statement charging that Israel was behind the fighting in Sidon. The leaders supported Lebanon's alliance with Syria and its membership in the Arab world.

Christian rebel commanders have scoffed at the Phalangist Party's submissive attitude to President Amin Gemayel, who they think has veered too close to Syria and compromised security considerations and the independence of his own Christian community.

Moslem leaders, including Karami, Berri and Hoss, welcomed the nine-point statement..

Tension along Beirut's traditional midcity confrontation lines has risen over the last three days and three main crossings were closed to traffic because of sniping.