Prime Minister Menachem Begin acknowledged today that he had assumed "acts of revenge" would follow the assassination of Lebanese President-elect Bashir Gemayel, but he said he did not consider this a reason to prevent the Christian militias Gemayel had commanded from entering the Palestinian refugee camps of West Beirut.

Begin, testifying before the board that is investigating Israel's role in the September massacre of hundreds of Palestinian refugees, acknowledged that he had mentioned the possibility of widespread bloodshed to both U.S. envoy Morris Draper and the Israeli Army chief of staff, Lt. Gen. Rafael Eitan, before the massacre occurred.

In the 45-minute public hearing, the commission also disclosed -- and Begin confirmed -- that during a Cabinet meeting on Sept. 16, the night the Christian Phalangist militia units entered the refugee camps, Eitan had warned of a possible "outburst of revenge" in Beirut.

But Begin, asserting repeatedly that "it didn't occur to anyone that they would commit atrocities," said no one at the Cabinet meeting suggested that they reverse Defense Minister Ariel Sharon's order for the militia units to enter the Palestinian neighborhoods.

Begin, dressed in a dark suit and tie, leaned back in a chair and appeared relaxed throughout the questioning. He was flanked at a table facing the three-member panel by Dan Meridor, the Israeli Cabinet secretary, and Yechiel Kadishai, his close personal and political confidant.

Begin's appearance before the board attracted intense interest in Israel. He spoke in Hebrew, but an English transcript of his testimony was released tonight by the government press office.

The most pointed exchanges during the hearing concerned a central question in the investigation: If the Israeli Army was ordered into West Beirut to prevent chaos and bloodshed following Gemayel's assassination, as Begin and others have said, why was it not considered dangerous to expose the Palestinian refugees to Gemayel's armed followers?

Begin today was twice confronted with his own words of warning about likely Christian attacks on Moslems when he justified the Israeli Army's move into West Beirut. He was asked in effect why the same possibility had not occurred to him when he learned that the Phalangist militiamen had been allowed into the refugee camps.

The tone of the questioning today, and of others in earlier public sessions, suggested that the commission members may have already concluded that the government was at least negligent in going through with a secret plan to use the Phalangist units in West Beirut despite the Gemayel assassination and its likely impact on his followers.

Gemayel was killed on Sept. 14, and at dawn the next day the Israeli Army began to move into West Beirut. Later on Sept. 15, Begin explained the decision to Draper at a meeting, the minutes of which the inquiry board has obtained.

Begin, asked to read from the minutes, quoted himself as telling Draper, "We did it to make sure that certain possible events be prevented. We were afraid lest there be bloodshed, even during the night."

Begin also said, according to the minutes, that "the Phalange are behaving well" and that Israel had confidence in the Phalangist commander. "But about the others, who knows?" Begin said.

Asked by Israeli Supreme Court Justice Aharon Barak, a member of the commission, "Who will shed whose blood?" Begin replied:

"When I referred to bloodshed I meant acts of revenge by Christians against the Moslems. Christians in general, not only the Phalangists."

"But also Phalangists?" Barak asked.

"Of course," Begin replied. "Their leader, the president-elect had been assassinated and we saw all the mourning which broke out regarding his killing. And so I assumed that there may be acts of revenge by all."

Barak asked: "Assuming that there may also be acts of revenge by the Phalangists -- their leader had been murdered -- under the circumstances was there no reason to ask whether it is proper to approve the Phalangists' entry into the camps?"

"Your honor," Begin replied, "I can only reiterate my previous statements that at that time not one of us could conceive that the Phalangists would not fight the terrorists. They entered in order to fight the terrorists and only the terrorists, and we considered them, in the light of experience, regular military units that would not deviate from their duty and not commit any atrocity."

According to earlier testimony by Sharon, the Israeli Cabinet secretly decided on June 15, the second week of the war in Lebanon, to use the Phalangist forces to fight the Palestine Liberation Organization guerrillas in West Beirut, seeing this as a way to hold down Israeli casualties. At 6 p.m. on Sept. 16, the Phalangist units entered Sabra and Shatila refugee camps with orders to root out the remaining PLO guerrillas there.

Begin testified today that Sharon issued the order on his own but had not exceeded the Cabinet authorization of June 15. He said he and other Cabinet ministers were first told of Sharon's order at a special Cabinet meeting on Sept. 16, about two hours after the militiamen had begun to enter the camps.

Begin confirmed, as others have testified, that during the meeting Deputy Prime Minister David Levy "expressed a very grave fear" about the decision. Questioning Begin about this meeting, commission member Yona Efrat, a reserve Army general, disclosed that Chief of Staff Eitan had told the Cabinet that "the second thing which will happen in Beirut is an outburst of revenge."

Paraphrasing Eitan's words from the Cabinet minutes, Efrat said, "Today they the Phalangists have already killed Druze there. It will be an unprecedented outburst. I already see in their eyes what they are waiting for."

Barak interjected, also drawing from Eitan's statement, "The whole organization is already sharpening their knives."

Begin, shown the minutes of the Cabinet meeting, said, "I can only state the fact that none of the ministers -- how was it put here at one of the meetings -- saw a red light go on against the background of these remarks."

Begin said the Cabinet merely "made a note" that the militia units already had entered the camps and that Levy's warning "did not arouse special attention."

In another exchange with Israeli Supreme Court Chief Justice Yitzhak Kahan, the chairman of the inquiry board, Begin was asked to read from the minutes of a Cabinet meeting on Sept. 19, the day after the massacre ended. During this meeting, Begin told his Cabinet colleagues why he and Sharon had ordered the Israeli Army to move into West Beirut following the Gemayel assassination.

"At night I spoke with the chief of staff as well," Begin said, quoting himself at the Cabinet meeting. "I told him that positions [in West Beirut] must be seized, precisely in order to protect the Moslems from Phalangist revenge. I could assume that after the murder of Bashir, the leader loved by them, they would take revenge on the Moslems."

Begin said he had meant to refer to "the Christians in general," but Kahan said, "But the thought that the Phalangists as part of the Christian forces may revenge Bashir's death indeed came up prior to the meeting of Sept. 16?"

"Yes, I cannot deny what is written," Begin replied.

The Sept. 19 Cabinet meeting produced a defiant statement charging that it was a "blood libel" to suggest Israel had any responsibility for the massacre. Ten days later, under intense domestic and international pressure, Begin agreed to the creation of the inquiry board.

The prime minister testified, as he has said earlier, that he first learned of the massacre at 5 p.m. Sept. 18 from a news report by the British Broadcasting Corp. Told by Barak that the commission has a report of a conversation between him and Eitan about the massacre the previous day, Begin said he could not remember it but would check.

Begin also denied a suggestion made by Sharon during his testimony that he may have learned of the massacre earlier than he has acknowledged. Sharon said Begin called Eitan the morning of Sept. 18 to ask about a report concerning Gaza Hospital, which is in Sabra, but Begin said today this must have been a "misunderstanding" by Sharon and that he had not spoken to Eitan that morning.

Begin's testimony was the briefest the board has heard in public. He is also the only witness not to be asked also to testify in private.