Fierce shelling today by the forces of rival Christian leaders Gen. Michel Aoun and Samir Geagea killed 20 people and wounded 41, police said. The barrages came after Aoun threatened to attack ships using ports controlled by Geagea. On Tuesday night, 11 people died and 21 were wounded in fighting. The casualties raised the overall toll to 972 killed and 2,564 wounded since the showdown for dominance of the Christian enclave north of Beirut broke out Jan. 30. Tuesday's deaths included Brig. Gen. Francois Zein, commander of the military academy in the Aoun-held Fayadiyeh suburb east of Beirut. Zein, 47, was killed when his office at the academy was hit in a barrage fired by Geagea's Lebanese Forces militiamen. Aoun's threat to attack ports controlled by Geagea was aimed at cutting off supplies for Geagea's Lebanese Forces militia. A communique issued by Aoun's headquarters warned ``local and international maritime agencies'' against serving any port along the 28-mile stretch of Mediterranean coast in the enclave. Geagea's headquarters said in a communique the warning was ``illegal because it was issued by an illegitimate side that usurped power.'' The communique pledged ``severe response to any attack on a civilian ship.'' Smoke billowed from east Beirut's district of Ashrafiyeh, Geagea's main stronghold, and the pinewoods of the Metn mountains, where Aoun's troops are entrenched. The thuds of exploding shells echoed across Beirut as radio stations interrupted regular programming to report on the developments and blare appeals from hospitals for blood donations to cope with the influx of casualties. Geagea controls Beirut harbor and the ports of Jounieh and Byblos north of the capital. Jounieh, 12{ miles from Beirut, is the terminal for a ferry and a hydrofoil from Cyprus. These vessels are the Christians' only outlet from their enclave, which is ringed on its land sides by hostile Syrian troops and their Moslem militia allies. Aoun's 19,000-man army controls only a three-mile strip of the Christian coast and he is not believed to have any working gunboats left from the heavy fighting with Geagea's militia of 6,000 fighters and thousands of reservists. He has heavy artillery in the mountains overlooking Beirut and the coast and has used it to fire on Geagea's flotilla of gunboats and armed speedboats. But the guns are not radar-controlled and the police spokesman noted: ``It will be extremely difficult for Aoun's gunners to hit mobile targets, especially speedboats.'' The communique said Aoun declared the coast between Christian east Beirut and the Madfoun bridge in north Lebanon a military operations zone. ``Sailing across it is banned to any vessel irrespective of its type or cargo,'' the communique said. Geagea ordered his flotilla to blockade Aoun's only sea outlet at Dbaye, north of Beirut, on April 23 in an effort to prevent the general shipping in weapons and ammunition. President Elias Hrawi and Prime Minister Salim Hoss drove to the Syrian capital, Damascus, for talks with President Hafez Assad on ways of implementing a peace plan to end Lebanon's 15-year-old Christian-Moslem civil war. Aoun has rejected the Arab League-brokered peace plan, endorsed by Lebanon's Parliament in the Saudi Arabian resort of Taif in October, because it does not guarantee the withdrawal of Syrian forces from Lebanon. Syria, the main power broker in Lebanon, maintains 40,000 troops in predominantly Moslem regions under a 1976 Arab League peacekeeping mandate. Aoun calls them an occupying army. He also refuses to recognize Hrawi's election to implement the peace plan. But Geagea has recognized Hrawi and cautiously accepted the peace plan, which calls for an equal distribution of power between Moslems and Christians.