President Reagan's surprise announcement of a Middle East peace initiative last night represents a major new U.S. commitment, the first of his administration, to break the deadlock on the Palestinian issue, which has long been at the center of the Arab-Israeli dispute.

As explained by senior administration officials, Reagan took the step without advance agreement by any of the major parties in the Middle East, including the leaders of Israel, Egypt, Jordan and Saudi Arabia, who were informed Tuesday by U.S. ambassadors of the plan which now has been made public.

Some of Reagan's proposals, especially the freeze on Jewish settlements on the West Bank and the eventual return of Arab territory, are anathema to Israeli Prime Minister Menachem Begin's government, which already has made known its deep opposition. Official sources here said Reagan's speech had been planned for tonight, but was moved up 24 hours at the last minute to preempt a strongly adverse public reaction from Israel.

Other Reagan statements, especially U.S. opposition to creation of a Palestinian state on the West Bank and Gaza, cut across longstanding positions of the Arab parties.

In addition, King Hussein of Jordan, who is assigned a key role in the peace plan outlined by Reagan, has made no commitment in advance to play such a role, a senior U.S. official said in a State Department briefing on the U.S.initiative. Hussein refused to take part in the Camp David process as forged by President Carter, but officials expressed the hope that Hussein will consider the Reagan plan different enough to change his mind.

The main hope, as outlined by Reagan, is that the dramatic events in Lebanon will provide a new reason and opportunity for the Middle East actors to rise above their bitter disputes in the common and overall interest of eventual peace.

But officials did not explain how they expect to convince nations and people involved to accept positions that had been opposed, in some cases adamantly, during earlier rounds of diplomacy.

Asked about the next step, the senior State Department official briefing reporters said this would await the "considered reaction" from Mideast parties, after they have had time for reflection.

There is no expressed expectation among informed officials that Begin will accept the key elements of Reagan's plans initially, and only a slim hope for eventual acceptance. Begin regards the West Bank as the legitimate "land of Israel," and only temporarily suspended his claim to it during the Camp David negotiating process at the urging of Carter.

The reactions of the Israeli public, including the Labor Party, which preceded Begin's Likud Bloc in power, give promise of being more positive in the long run, in the view of American officialdom. Some of the ideas expressed by Reagan, especially that of peace based on a territorial compromise with Jordan, had been adopted in the past by elements of earlier Israeli governments.

The Arab reactions are likely to take weeks or even longer to develop in authoritative fashion, according to U.S. specialists. One of the reasons for announcing a definite U.S. position now was to do so before an Arab summit conference, which is to begin next week in Fez, Morocco, and thus possibly affect the future direction of Arab leaders at a crucial moment.

Many of the positions taken by Reagan last night had been espoused by earlier American administrations at some stage of the tortuous Middle East bargaining.

What is new, and dramatically so, was bringing them together in a clear and definite statement by a president in a formal address intended to define the course of future negotiations.

What is most surprising, almost startling, is that Reagan, who previously had expressed only sketchy ideas about Mideast peace and had been considered unusually close to Israel, would take such an independent and detailed position.

Officials took pains last night to say the plan originated in two trips by then-secretary of state Alexander M. Haig Jr. in January to the Middle East in an effort to restart the stalled negotiations on Palestinian autonomy. At the end of the second trip Haig concluded that this could not be done before the rest of the Sinai was returned by Israel to Egypt on April 25.

In the spring the administration worked out a detailed scenario for moving ahead with the autonomy talks but, before this could implemented, Israel invaded Lebanon on June 6. As the battle raged, Haig resigned on June 25.

The replacement of Haig by George P. Shultz in the top diplomatic job appears to have been a turning point. While considered a candidate for the same office before Inauguration Day, Shultz made it known privately and publicly that he could not agree with what seemed to be the strongly committed pro-Israeli views espoused by candidate Reagan.

At his first confirmation hearing July 13 Shultz made the much-noted statement that, "The crisis in Lebanon makes painfully and totally clear a central reality of the Middle East: the legitimate needs and problems of the Palestinian people must be addressed and resolved -- urgently and in all their dimensions."

However, he gave no idea then or in following days how he would go about this, other than to say it should be through the process set in motion at Camp David.

Shultz began to give shape to definite ideas in a day-long meeting on the Middle East with former secretary of state Henry A. Kissinger and other outside and governmental experts July 17, the day after being sworn in.

With the military action at a high point around Beirut, the internal discussions continued about a broad peace initiative to be unveiled when the crisis was over.

The crucial meeting, according to an administration official, was a lengthy discussion involving Reagan, Shultz and several other senior officials at Camp David on Aug. 14. The main lines of the plan unveiled last night were decided then, the official said.

Shultz was instructed to test the waters on Capitol Hill and elsewhere for these ideas. He did so in closed-door meetings with Senate Republican leaders, the Senate Foreign Relations Committee and the House Foreign Affairs Committee in the week following the Camp David session. Last Thursday he carried the discussion further in a private meeting with leaders of the American Jewish community.

According to State Department officials, the views of Middle East parties were tapped through Washington visits by parties in the area and inquiries from U.S. diplomats abroad.

Late last week, the U.S. ambassadors to Egypt and Saudi Arabia, Alfred L. Atherton Jr. and Richard W. Murphy, respectively, participated in unpublicized talks here with Shultz. They returned to their posts Monday to present advance word of the Washington decisions.

The positions finally recommended by Shultz, and announced by Reagan last night, bear startling resemblance to views stated in recent months by Kissinger.

In a Washington Post article on June 16, Kissinger stated six principles for overcoming the impasse on the Palestinian question, including an interim territorial compromise on the West Bank as the basis for autonomy there and a leading role for Jordan in West Bank negotiations.

Moreover, Kissinger said progress could be made "only if the United States spells out what it understands by autonomy in an interim agreement."

Reagan went a long way toward spelling out such understandings last night, and apparently is prepared to go into further detail if expanded negotiations on the issue begin.

It is clear from the large number of disputed issues, and the likely uproar in several quarters of the Middle East, that the U.S. plan faces great pitfalls.

Added to all of the other problems is the unresolved situation in Lebanon, where Syrian and Israeli forces continue to face each other following the departure of the Palestine Liberation Organization fighters.

The senior State Department briefer said the administration hopes the Palestinian diplomacy can proceed before the Syrian and Israeli withdrawals are completed. Nevertheless, an eruption of violence between those two armies could divert the attention of policy makers from the peace table to the battlefield at any moment.

For all the admitted difficulties of the Reagan initiative, the dangers of failing to act were even greater, in the view of officials working on Middle East affairs.

The autonomy process was deadlocked, with no way in sight to break the impasse under the existing framework. Egypt, which reluctantly had accepted the role of bargaining for the Palestinians after the Camp David accords, has become both less willing and less able to do so.

Israel, in a sign of its own lack of confidence in the process, had begun increasingly to suggest, as Defense Minister Ariel Sharon did here last week, that it could make its own deal on the West Bank with Palestinians it chooses.

It is still very uncertain how the dispersal of the PLO leadership and military units, and the other results of the fighting in Lebanon, will affect the situation in Middle East.

But, according to U.S. diplomats who shaped the Reagan plan, it is entirely clear that positive and lasting progress from the new era can only result from a U.S. leading role, such as the one Reagan began to assert last night.