Israeli security agents on hand for the arrival of an El Al flight with 160 passengers opened fire on four Palestinian terrorists here today and prevented what could have been a major bloodbath.

The Israelis shot, then seized one heavily armed terrorist and captured a second. Belgian police joined the shootout but were unable to find another man and woman said to be among the attackers.

The terrorists, armed with fragmentation grenades and Soviet-made submachine gun, hurled two light hand grenades into the crowded terminal from a balcony, wounding what officials said was "about a dozen persons." none seriously. The victims, all Belgians, were taken to hospitals in Brussels, 10 miles from the airport.

Belgian police joined the attack on the terrorists but Defense Minister Paul vanden Boeynants-who rushed to the airport as soon as he heard of the attack-said. "The first shots at the terrorists were fired by El Al security agents who happened to be there."

While police said one or two other terrorists were believed to have escaped, most eyewitnesses mentioned two, one of thema woman.

Authorities said the captured terrorists carried bogus Lebanese passports, and said the El Al plane was their target. They reportedly declared they were from a new group of the Palestine Liberation Organization called "Black March"-for the month when the Egyptian-Israeli treaty was signed.

Belgian state radio said the terrorists belonged to the Marxist radical Popular Front for the Liberation of Palestine, a group that broke away from the PLO in 1974.

In Beirut, the PLO disclaimed any knowledge of the attack.

Last month the PLO spoke of reviving the Black September terrorist group to punish Egypt and Israel for signing the peace treaty. The Black september group carried out the massacre of Israeli athletes at the Munich Olympics in 1972.

Airport authorities identified the attackers as being from a new group called "Black March," named for the month the Arab-Israeli treaty was signed. Police did not release the names of the terrorists but said they carried bogus Lebanese passports.

Authorities said the terrorists carried written demands that they be flown out of Belgium and would have presented if their hijacking attempt had succeeded.

The plane, flight 334, had arrived from Tel Aviv via Vienna and already had loaded passengers for a return flight when the terrorists struck.

Airport authorities said the usual El Al security precaution of parking its planes a good distance from the terminal foiled the apparent intent to seize the aircraft. Le Al planes are considered a choice target by Palestinian terrorists.

Police closed the roads leading to the airport for three hours, stranding hundreds of persons on Easter Monday, a holiday in Belgium.

Police then raided the Brussels residences of well-known PLO sympathizers, informed sources said.

In Tel. Avis, Modechai Hod, managing director of the airline, said there was no evidence that the attack was specifically aimed at El Al passengers.

"It wasn't an El Al area [of the terminal] and the people weren't El Al passengers," he said.

A belgian businessman in the terminal during the 2 p.m. attack said, "I heard two explosions behind me. Then there was smoke everywhere and everybody hit the floor9" He was at the airport to pick up his children from the flight.

Mario Possamai, a journalist with the Canadian Press news agency who had just arrived on a flight from Toronto, said he heard a burst of gunfire and saw smoke coming from the hall leading to the arrival area.

He said heavily armed police were swarming through the area when he got there, and that there were several appeals for doctors over the public address system. Several ambulances arrived at the terminal as did two large field hospital vans. CAPTION: Picture 1, KARIM SANJABI . . . prominent foe of shah; Picture 2, Belgian police blockade the main hall of the Brussels airport after a terrorist attack in which the man at right was slightly wounded. AP; Picture 3, Belgian police blockade the main hall of the Brussels airport after a terrorist attack in which the man at right was slightly wounded. UPI