BEIRUT, SEPT. 21 -- President Elias Hrawi today signed constitutional reforms designed to give Moslems an equal share in power with long-dominant Christians in a bid to resolve Lebanon's 15-year civil war.

Hrawi's move was aimed at setting in motion a peace plan stalled by opposition from Gen. Michel Aoun, the Christian army commander. Hrawi, a Christian, urged Aoun to "peacefully join the process of ending the civil war."

"Otherwise, I will be forced to make the bitter decision of resorting to an imperative surgical operation" to crush Aoun's forces, Hrawi said at a signing ceremony. The ceremony in Moslem west Beirut was attended by Prime Minister Selim Hoss, a Sunni Moslem, and legislative Speaker Hussein Husseini, a Shiite Moslem.

There was no immediate response from Aoun, whose dwindling force of about 15,000 troops holds pockets in east Beirut and nearby mountains.

Hrawi said a new government comprising representatives of all factions would soon be formed "to end the civil war, dissolve various militias, reestablish state authority over Lebanon's entire territory and shore up the battered economy."

Maronites, the largest Christian sect, have held top posts in the government, army, judiciary and central bank since Lebanon gained independence from France in 1943. At that time, Christians were thought to be the majority, but Moslems now are believed to account for 55 percent of Lebanon's 4 million people.

Although many Lebanese express skepticism about Hrawi's ability to remove Aoun and carry out reforms, the peace plan, mediated by the Arab League, was widely viewed as providing a mechanism for the first time to resolve the civil war.

"The Lebanese have a real chance to patch up now," Arab League mediator Lakhdar Ibrahimi said. "They can make the plan work or wreck and lose the chance of salvation forever."

The reforms Hrawi signed are embodied in constitutional amendments that vest executive powers in Lebanon's Council of Ministers, stripping the president of the exclusive right to implement major decisions. The cabinet and legislature will contain equal numbers of Moslems and Christians, eliminating the 6-to-5 edge Christians held in both bodies. However, a formula of having a Maronite president, a Sunni prime minister and a Shiite speaker will remain.

The reforms were worked out by Lebanese legislators who met in Taif, Saudi Arabia, in August 1989 to endorse the Arab League plan. Aoun rejected the accord and has refused to recognize the Syrian-backed Hrawi, who was elected in November to oversee implementation of the peace plan. Aoun contends the peace accord does not contain guarantees that the Syrians, whom he considers an occupation army, will withdraw.