A police guard stationed on a balcony overlooking the El Al check-in area at this city's international airport distracted three terrorists for crucial seconds in their assault on passengers lining up for the Israeli airline's flight to Tel Aviv this morning, probably saving countless lives, police officials here said tonight.

An initial exchange of gunfire between the terrorists and the guard sent passengers and staff diving for cover and allowed at least a dozen other armed security and police personnel in the area time to react and begin firing their weapons.

As the terrorists rolled grenades across the floor and sprayed machine-gun fire through the check-in area, they were met with a barrage of return fire that drove them back down the stairway up which they had come. One guard, part of an extra security contingent El Al hires for all its flights here, kicked one grenade after them, and it exploded on the stairs as they backed out of the building still shooting.

The terrorists fled in a commandeered car, chased by still-shooting police who stopped them about one mile away. One was dead, the other two seriously wounded. None has been identified, although police here describe them as "Arab-appearing" and say one told them only that he was a Lebanese who had traveled on a Jordanian passport.

In their wake, the terrorists left a scene of bloody carnage, broken glass and bullet holes in virtually every wall, window and counter of one end of the airport departure area. Although the lights still blinked on a Christmas tree across from the counter, many of the ornaments were shattered. Amid scattered luggage and pools of blood, one passenger lay dead, 50-year-old Austrian Prof. Ekhart Karner. Another, among at least 47 persons wounded, died later in a nearby hospital. He was identified by police as a Viennese resident with the surname Gana, who was believed to be an Israeli citizen.

The drama at Schwechat Airport, southeast of the city, began just after 9 a.m., 90 minutes before the flight to Tel Aviv was due to depart with a passenger list made up primarily of Austrian and Israeli citizens. During holiday periods, El Al increases its usual two or three flights per week from Vienna to four. Passengers for all Israeli flights must go through special security checks of their luggage and body frisking as a precaution against terrorist attacks.

The normal contingent of 30 Austrian federal police was on hand in the airport, many of them armed with submachine guns, and at least a half dozen of them were in the vicinity of the two counters at the far end of the second-floor departure area set aside for El Al. Also in the vicinity were plainclothed, armed Austrian police.

Checking passenger luggage behind three long, metal tables placed in a U shape adjacent to the check-in counter were at least three security agents, part of the force hired by El Al for all its flights. A separate line of passengers, waiting to check in for a flight to Greece, stood at a nearby counter.

The three terrorists, carrying Kalashnikov automatic rifles, apparently entered the terminal building through a door at the end of the first-floor arrivals section. Police speculated that they had come by train on the rapid transit system from central Vienna.

Just inside the ground-floor entrance is a wide stairway leading directly up to the El Al check-in area, and police said it was clear that the men knew in advance where they were headed. As they ran up the stairs brandishing their weapons, however, they apparently spotted the guard on the balcony above the El Al counter, and began firing up at him. One shot went into a window behind the guard, several into a flight-listing board and a number shattered a metal-topped glass railing in front of him.

Lt. Col. Herbert Zuleuer, chief of the federal police contingent assigned to the airport, said tonight in an interview that this "first wave of attack at the balcony" gave other security officers a chance to respond, so that seconds later, "by the time the second wave came at the passengers," the officers already had begun firing back.

The terrorists sent three grenades across the floor. One exploded in the hall near the Christmas tree, leaving a shallow, foot-wide hole in the black and white stone floor. Another failed to explode.

The terrorists began shooting wildly around the area, bullets hitting people, peppering columns and walls, shattering glass and flying into a nearby restaurant area and hairdressers' salon.

Some passengers and staff pushed over the metal tables, trying to hide behind them or pieces of luggage on the floor, while others jumped behind the counters. Austrian tourist Reinhard Schubert, 39, told United Press International that he saw "hand grenades like black balls rolling through the hall. I saw two or three people lying on the floor. One was bleeding from the throat. I hit the floor and crept away for shelter."

The return gunfire eventually drove the terrorists back down the stairs, where the El Al guard kicked the third grenade after them. As the terrorists ran back out the first-floor door, they discarded two of their rifles, later found by police to be out of ammunition.

Outside the door, on a nearby ramp leading up from an underground parking area, the three stopped a departing Mercedes, pulled its driver out and jumped in, speeding out the airport exit. As they left, a guard on the terminal roof "emptied the magazine" of his automatic weapon at the car, said police inspector Peter Lehner.

Airport police jumped into three other cars in pursuit. Just outside the airport gate, the Mercedes stopped at a gas station, where one terrorist leaned out to pitch another grenade at the pursuing police. It exploded harmlessly in the street.

The chase went on for about a mile, on a highway leading east-southeast. Police, who had closed the road from the airport, continued firing at the fleeing Mercedes, finally disabling it.

As the Mercedes crashed into a hedge, at least one of the terrorists, apparently wounded, jumped out and attempted to stop two cars traveling from the other direction.

Zuleuer said that when police reached them, one of the terrorists already was dead. One of the others, shot in the midsection, was unconscious. The other, shot in the back and lungs, was only barely conscious. None of the three carried any identification.