BEIRUT, AUG. 21 -- Lebanon's National Assembly approved constitutional reforms today that give the country's Moslem majority more say in the Christian-dominated political system, addressing one of the Moslems' main grievances in the country's 15-year-old civil war.

A political source said the move, although strongly opposed by Christian rightists, was a breakthrough in the political stalemate and presented a "real chance to end the civil war."

It was the first time since Lebanon's independence in 1943 -- when Christians were thought to comprise a majority of the population -- that fundamental amendments to the constitution were introduced. Forty-eight of 51 legislators who gathered at the assembly building voted to approve the change.

Just 70 of the 99 assembly members elected in 1972 to four-year terms have survived. Violence since then has prohibited another election.

The amendments included a shift in power from the president to the cabinet as a whole. According to tradition, the president must be a Maronite Christian, the prime minister a Sunni Moslem and the speaker of the assembly a Shiite Moslem.

The reforms also increase the number of legislators to 108 on a half-Moslem, half-Christian basis. Seats in the assembly had been divided on a ratio of six Christians to five Moslems.

The reforms were in line with a peace plan drawn up last year by Lebanese lawmakers meeting in Taif, Saudi Arabia, under the auspices of the Arab League. The plan also stipulated the withdrawal of Syria's 40,000 troops from most parts of the country within two years and called for the disbanding of all Lebanon's warring militias in six months.

The pact is vehemently opposed by defiant Christian leader Gen. Michel Aoun, who is holding out with his troops in the Christian enclave of Beirut. He also refuses to submit to the authority of the internationally backed president, Elias Hrawi.