Prime Minister Menachen Begin declared today that Israel will concentrate its preemptive bombing attacks in Lebanon on the political headquarters of Palestinian guerrilla organizations in densely populated civilian centers as well as military bases in outlying areas.

In a toughly worded statement issed to coincide with today's attacks on Beirut, Begin said that if the lives of Lebanese civilians are endangered, the responsibility should rest with the Palestine Liberation Organization.

In the past, Begin said, Israeli strikes in Lebanon were designed to avoid hitting civilian population centers and were directed only at "military targets of the terrorists." In contrast, he said, the guerrilla organizations aimed their attacks exclusively against Israeli civilians.

Begin cited Wednesday's massive Katyusha rocket attack on the Israeli resort town of Nahariya, in which three civilians were killed and 25 wounded, adding, "Under no circumstances will we tolerate such attacks and their consequences."

"Now, too, we will not intentionally direct our fire against the civilian population," he added. "We shall, however, continue to attack terrorist bases and headquarters, even if they are purposefully located in the vicinity of or within civilian concentrations. Responsibility shall fall on those who seek immunity for themselves by knowingly endangering civilians. We shall give the enemy no rest until we have put an end to his bloody rampage and peace will reign between Israel and Lebanon."

Sources in the prime minister's office said Begin, who is also defense minister, gave final approval to attack the central Beirut headquarters of two PLO groups after Palestinian guerrillas in southern Lebanon resumed their rocket attacks this morning on Nahariya and Kiryat Shemona, an Israeli development town in the Galilee panhandle. A Katyusha rocket hit a maternity hospital in Nahariya, slighting injuring two woman.

It was the sixth day in a week in which the Israeli Air Force had pounded Palestinian positions in Lebanon. Israeli jets returned this afternoon, bombing and strafing a PLO Fatah headquarters in the coastal city of Tyre and destroying three bridges spanning the Zaharani and Litani rivers.

Deputy Israeli Defense Minister Mordechai Zippori echoed Begin's warning that the political infrastructure of the PLO would now be a prime target for Israeli strikes.

"You have to deal with this thing at the top. Until now, we have only dealt with the lower level of the PLO fighting units. Now we decided we have to go to the top and attack the headquarters . . .," Zippori said.

Israeliu Foreign Minister Yitzhak Shamir said tonight that the air strikes were aimed "expressly and specifically at terrorits concentrations" and that Syrian troops in Lebanon were not intended targets.

While Syrian soldiers stationed in the vicinity of Palestinian guerrilla positions "may have been hurt," Shamir said, "this is not Israel's intention." Shamir said he hoped that future Israeli operations in Lebanon would not lead to a deterioration of the Israeli-Syrian situation.

"I don't see any connection between these operations and the mission of Mr. Phillip Habib," Shamir said, referring to the U.S. special envoy who is attempting to defuse the crisis over deployment of Syrian missiles in Lebanon.

"Until now, the Syrians have not interfered with our confrontation with the terrorist organizations. I hope this will continue. . . . We hope we will not have any further complications," Shamir said.

Since Israel's openly declared preemptive strike policy in Lebanon was put into effect in 1979 by then-defense minister Ezer Weizman, Israeli forces have concentrated air and ground strikes against relatively small guerrilla training and staging bases in southern Lebanon.

The oft-declared purpose of those raids, Begin and other Israeli leaders made it clear, was to constantly disrupt the military infrastructure of the guerrilla organizations and keep it an off balance that it would be unable to launch cross-border terrorist attacks into Israel.

The strategy appeared to be successful, since the last infiltration completed by a PLO terrorist squad occurred in April 1980 in the border kibbutz of Misgav Am, where two Israelis were killed. Attempts by the PLO to launch cross-border raids -- at least those made public -- have also fallen off sharply as a result of the preemptive strikes.

But an Israeli air strike July 10 against a convoy of vehicles with Katyusha rockets mounted on them moving across the Zaharani River, the first preemptive raid in more than a month, triggered a new round of Palestinian rocket and artillery attacks and a series of Israeli counterstrikes.

Whether today's massive attacks in Beirut and Tyre would create a lull in the spiral of violence was uncertain, Zippori said, "I hope that [it] will stop the attacks and counterattacks, although I can't be sure that from time to time they will not not succeed in landing shells on some of our settlements. . . . But, basically, we shall act, let us say, in the most serious way to stop it and prevent it."

Former foreign minister Moshe Dayan, however, said that while the attacks on Beirut may have some temporary success, "everyone will have to have the last word" and the cycle will continue.

Dayan, in a radio interview, said that the reciprocal attacks probably would not end until the United States or another third party mediates a cease-fire between Israel and the PLO.

The idea of a political settlement was also advanced by former Army chief of staff Mordechai Gur, who urged the "combined efforts of the Americans, the Syrians and the Lebanese government" to arrange a cease-fire.

Noting that the Syrians control the highways in Lebanon from the north to the south and have significant influence in the headquarters of the guerrilla organizations, Gur said, "There is no doubt that once the Syrians will realize that this [a cease-fire] must be done, that otherwise things might happen that nobody, including the Syrians, wants, the sooner they will do it."

In the background of the air strikes was a domestic political problem that had become increasingly acute for Begin, who had repeatedly promised during the recent national election campaign that if his Likud Party were returned to power, rockets fired by Palestinian guerrillas in Lebanon would cease to fall on Kiryat Shemona and other northern Israel border towns.

Following two days of heavy bombardment, some residents of Kiryat Shemona and Nahariya, both of which are located about 10 miles south of the Lebanese border, had begun to complain that Begin was not keeping his promise.

In background papers issued to coincide with the renewed air strikes in Lebanon, the government cited a series of legal precedents for to self-defense and retaliatory military operations, some dating back over more than 100 years.

The Israeli Army command spokesman's office cited the case in 1837 in which British soldiers crossed the U.S. border from Canada and destroyed the Caroline, an American vessel that had been used to supply insurgents in Canada.

Another government position paper noted 1916 incursions by the United States into Mexico to attack maruaders led by Pancho Villa, and the U.S. intervention in the Suez Canal in 1956.

Those incidents, according to the Israeli government, justify Israel's legal position in conducting ground and air operations in Lebanon.