A wobbly truce went into effect in the Beirut area today after Syria stepped in to halt the heaviest round of sectarian fighting in more than a year, and a special security committee agreed on the deployment of Syrian observers to monitor confrontation lines.

In what some saw as another sign of hope, Prime Minister Rashid Karami's "national unity" Cabinet, paralyzed for months by opposition from radical Moslem leaders and deteriorating security conditions in Lebanon, met today with President Amin Gemayel for the first time since April.

But two key members -- Nabih Berri, leader of the Shiite Moslem Amal militia, and his Druze ally, Walid Jumblatt -- continued their year-old boycott of the Cabinet, raising questions about its effectiveness.

Berri, whose west Beirut neighborhood has been among the hardest hit in the past four days of massive shelling, also expressed skepticism about the Syrian-arranged cease-fire and delayed giving it his support.

Shelling continued sporadically for several hours after the cease-fire was to have taken effect at 1 p.m., but by 8 p.m. the guns were mostly silent throughout the city. Police said at least 21 persons had been killed and nearly 50 wounded in today's hostilities, bringing the casualty toll from the fighting and a series of car bombings to 301 dead and nearly 1,000 wounded since Aug. 10.

Syria's chief of military intelligence in Lebanon, Brig. Ghazi Kanaan, convened the meeting of the special security committee, including representatives of the Lebanese Army and the Christian, Druze and Shiite Moslem militias in Anjar in the Syrian-controlled Bekaa Valley.

The committee prohibited the use of heavy arms against civilian areas and agreed on the deployment of Syrian military observers to supervise flashpoints in Beirut and the nearby mountains, but it gave no details on the numbers.

Berri, however, complained that the military observers would be able to do little more than count the blows and report on them. He insisted that more guarantees are needed before his group commits itself to the cease-fire.

Gemayel, at the Cabinet meeting thanked Syrian President Hafez Assad and Vice President Abdul Halim Khaddam for their efforts in dealing with the crisis. Despite recent speculation that Gemayel's days are numbered and his increasing isolation in both Christian and Moslem camps, it appeared Syria is still backing him.