The Lebanese Army launched preemptive air and ground attacks today to strengthen its hold on a strategic ridgeline in the mountains overlooking the presidential palace and the capital.

Two Lebanese Army Hawker Hunter jets were knocked out in the fighting, which shattered the relative calm of the past several days. There was also a resumption of widespread shelling, with rounds falling in Beirut's Christian neighborhoods, on the U.S. Marine compound, near the Lebanese Defense Ministry, the presidential palace and the British and temporary American embassies.

The level of tension here was further raised as Israeli fighter planes flew over Israeli-controlled areas in southern Lebanon and Syrian jets flew over Syrian-occupied territory near the northern border. Neither side's planes got involved in today's fighting.

The Lebanese Army attack came early this morning, only hours after a meeting of top Lebanese government officials with U.S. and Saudi Arabian envoys to discuss cease-fire arrangements, a session that had lifted flagging hopes here for an end to the fighting.

Military officials said the preemptive strikes were to thwart efforts by Druze militiamen and their Palestinian allies to capture the vital heights held by the Army at Suq al Gharb. The position is considered pivotal because it is the link to the two major Druze concentrations and because it affords a clear line of sight on the palace and the capital.

The Druze began a counterattack this evening.

"We are night fighters," a spokesman for the Druze's Progressive Socialist Party militia said. "The day is for the Army and the night is for us."

Journalists in areas near the fighting earlier today said the Army had taken control of heights around Suq al Gharb. Army communiques said Army units had destroyed hostile artillery positions and prevented tanks from advancing on their positions.

A Lebanese Army fighter was shot down over Suq al Gharb and the pilot parachuted to safety over the Mediterranean, where he was rescued by a helicopter from the U.S. carrier Eisenhower. The pilot of the other plane hit in the fighting managed to land his craft in Cyprus.

The streets of Christian east Beirut were virtually deserted today, except for the occasional speeding car taking passengers to somewhat safer areas. The pounding of Christian neighborhoods could be heard into the evening.

Four mortar rounds fell in the U.S. Marine compound around Beirut International Airport but no one was injured. There were also no casualties when three rockets fell near the British Embassy--which turned over a wing to U.S. diplomats after the April bombing of the American Embassy--and a nearby apartment building on the seafront which had been turned into makeshift quarters for other U.S. Embassy operations. One of the shells struck a building on the American University of Beirut campus in between. Another fell into the sea.

Embassy staffers said the rounds appeared to come from areas northeast of them, which are Christian militia strongholds.

Western military sources said President Amin Gemayel and Army Commander Gen. Ibrahim Tannous decided late last night to launch the attack today because of the infiltration of antigovernment forces into routes leading into Suq al Gharb.

Lebanese intelligence sources claimed that intercepted radio transmissions indicated about 1,200 fighters were infiltrating, although the western sources said they doubted the actual number was much more than one-third of that.

Gemayel and Tannous were said by the western sources to be concerned that the Army operation would complicate the cease-fire negotiations but felt they had to go ahead because the threat was so great.

One participant in Thursday night's meeting on cease-fire arrangements spoke optimistically to a colleague this morning about the progress that was made, unaware that the Army's preemptive attack had begun three hours earlier.

Lebanese government officials, Druze chieftain Walid Jumblatt and his Syrian allies are agreed on the principle of a cease-fire followed by a national reconciliation conference to bring warring Lebanese factions to an accord.

Jumblatt has publicly called for the withdrawal of the Army from positions in the mountains as part of any cease-fire arrangement, but Gemayel has insisted that the Army stay. Other differences reportedly have been over who would be invited to a reconciliation conference and where it would be held.

Tunis, Riyadh and the villas and mansions of various Lebanese chieftains have been considered and rejected by one side or another.

Publicly, Lebanese government officials have refused Syria's demand that an emissary of the Damascus regime be permitted to participate as an observer in the reconciliation conference. However, two sources familiar with the negotiations said that privately, the Lebanese have tentatively consented to allow both Syrian and Saudi observers.

According to the sources, last night's meeting, which was attended by Robert C. McFarlane, President Reagan's special Middle East envoy, and Saudi Prince Bandar Sultan, attempted to arrive at a compromise on the Army issue by proposing that United Nations observers supervise the cease-fire and that the Army stay in its present positions and not attempt to expand the small territory it controls in the mountains.

This afternoon, Army spokesmen were claimimg they had taken Eitat, a Druze stronghold on the ridgeline across from Suq al Gharb, although later reports described the Army as still advancing toward the town.

Prince Bandar met with Syrian Foreign Minister Abdel Halim Khaddam and Druze leader Jumblatt in Damascus today before leaving for Saudi Arabia. Asked if he would return, Bandar replied, "I do not know yet."