President Reagan, expressing "outrage and revulsion" about the massacre of Palestinian civilians in Beirut, charged yesterday that, in effect, Israeli actions were responsible for the killings. He demanded that Israeli military forces withdraw immediately from West Beirut.

Using what appeared to be the angriest language ever directed at Israel by an American president, Reagan stopped short of saying explicitly that he holds Israeli Prime Minister Menachem Begin's government responsible for the killings, which apparently were carried out by Lebanese Christian militiamen. But his words left no doubt about where he feels the blame should be placed.

"During the negotiations leading to the PLO [Palestine Liberation Organization] withdrawal from Beirut, we were assured that Israeli forces would not enter West Beirut," Reagan recalled in a statement issued by the White House. It added:

"We also understood that following withdrawal, Lebanese army units would establish control over the city. They were thwarted in this effort by the Israeli occupation that took place beginning on Wednesday.

"We strongly opposed Israel's move into West Beirut following the assassination of President-elect Bashir Gemayel both because we believed it wrong in principle and for fear that it would provoke further fighting.

"Israel, by yesterday [Friday] in military control of Beirut, claimed that its moves would prevent the kind of tragedy which has now occurred."

The president's statement continued: "We have today summoned the Israeli ambassador to demand that the Israeli government immediately withdraw its forces from West Beirut to the positions occupied on Sept. 14.

"We also expect Israel thereafter to commence serious negotiations which will first lead to the earliest possible disengagement of Israeli forces from Beirut and, second, to an agreed framework for the early withdrawal of all foreign forces from Beirut."

Late yesterday afternoon, Ambassador Moshe Arens, who had been observing Rosh Hashana, the Jewish new year, went to the State Department where Secretary of State George P. Shultz gave him what U.S. officials described as a formal and strongly worded withdrawal demand.

After the half-hour meeting, Arens told reporters: "I'm sorry. I have no statement to make."

Following Reagan's statement, a senior U.S. official, speaking on the condition that he not be identified, elaborated on the reasons for the strong American stance. Taking the same tack as the president, he refused to say explicitly that Israel is responsible for the massacre.

"We don't know who has been in there pulling the trigger," the official said. He added that reports from the scene, while confused and contradictory, gave the impression that the killings were the work of Christian forces commanded by Maj. Saad Haddad, widely regarded as Israeli-controlled, and to a lesser extent Christian Phalangist followers of the assassinated Gemayel.

The official also stressed that the United States has no evidence to indicate that Israeli troops participated in the massacre or stood by and did nothing to prevent it. But he noted pointedly:

"We do know that we were told by the Israelis that they were in security control of the whole area. Their justification was that this military presence would prevent the kinds of things that happened."

"These are the facts," the official asserted, adding: "Yes, we would like the immediate withdrawal of the Israeli Defense Force from Beirut. It has not been a stabilizing influence . . . .

"I cannot tell you what would have happened if the Israeli army did not go into Beirut. I can only tell you that they did go in and there were these tragic developments. We think it's time to let the Lebanese control their own city."

The official said that Washington has only fragmentary accounts of the deaths and does not know if the toll will be as high as 1,000, as some reports from Beirut have suggested.

He said that U.S. Embassy officials in Beirut were able to count "approximately 50 bodies, including women and children" in one camp and that information from other western diplomats regarded as reliable "takes it up to about 300."

The official took strong exception to reports from Israel in recent days contending that the Begin government never promised not to enter West Beirut. On the contrary, he insisted, when special U.S. envoy Philip C. Habib was negotiating the PLO withdrawal, the Israeli government -- from Begin down -- repeatedly gave assurances that it would not move against West Beirut and that civilians would be protected from harm.

He refused to speculate whether an Israeli refusal to pull back from Beirut might cause Reagan to withhold U.S. military aid from the Jewish state. "We'll cross that bridge when we come to it," he said.