Reza Pahlavi, eldest son of the late deposed Iranian monarch, proclaimed himself the new shah of Iran here today as he celebrated his 20th birthday.

In a quiet ceremony at Hubbeh Palace unattended by any family members, Egyptians or any other officials, the young exile read an 11-minute message to Iranians in his native language of Persian declaring "my readiness to accept full responsibility as the lawful king of Iran."

He immediately called upon "all patriotic groups inside and outside" Iran to rally behind him in "the common cause" of restoring him to the Peacock Throne and ending the "nightmare" he said his country was now passing through under the Islamic revolution of Ayatollah Ruhollah Khomeini.

[In Washington, State Department spokesman John Trattner said the United States had no intentions of supporting Reza Pahlavi, adding that "we accept the result of the April referendum and accept the government of Iran as the legally constituted government in Iran."]

In his message, Reza gave no indication how he thought this might be done, but in an interview with a French television station yesterday he asserted confidently that "the Iranian people will rise, I am sure of it, and sweep away this faithless, lawless regime" of the ayatollah. "Those are the living forces on which I lean."

With Iranians inside the country now united as never before in the struggle to defeat the invading Iraqi forces and the opposition outside bitterly divided into myriad factions, Western observers here saw little immediate prospect for such an uprising. In fact, the absence of even family members at the Kubbeh Palace ceremony today seemed all too symbolic of the young, exiled pretender's personal and political isolation at present.

The only outsiders allowed to witness the event were an Egyptian and French television team and a single photographer from the Associated Press and United Press International.

A spokesman for Reza Pahlavi said the ceremony was deliberately kept low-key and the press exclused because of the "delicate international situation," a reference both to the Iranian-Iraqi war and the possible release of the American hostages in Iran.

Dressed in a dark blue suit and tie and having seemingly rehearsed his brief speech, the young pretender delivered his statement seated at a marble-topped table in a first-floor alcove of the 400-room palace given over to the late shah and his family when they came to take up exile here last March. Behind him to the left was a picture of his late father Mohammad Reza Pahlavi and to the right the flag of imperial Iran.

His appeal was carefully couched in both nationalistic and religious terms. He spoke of establishing a new monarchical order based on "the teachings of true Islam" and kissed a copy of the Koran laying beside him at the end of his statement as well as playing a recording of the Iranian national anthem. It was not immediately clear how his message would be relayed to Iranians inside their country located some 1,200 miles away from Cairo. Earlier, however, his spokesman said they would probably rely on the use of recordings and cassettes smuggled into Iran, just as the ayatollah did so successfully in his campaign to overthrow the late shah.

In his message, he said his official title would be Reza Shah the Second, and he promised to "faithfully observe and uphold" the imperial constitution dating back to 1906 and scrapped by the Islamic revolution in favor of a fundamentalist Islamic one in late 1979.

The youthful Pahlavi said he was well aware he was undertaking this commitment "at one of the darkest periods in the history of Iran."

"There has been an assault from within upon the foundations of our nationhood. Furthermore, as the result of social anarchy, political isolation and economic paralysis we have lost our place in the family of nations and have suffered an act of external aggression against our territorial integrity," he said. But, he added, "There is, I am sure, light beyond the darkness and deep in your hearts you may be confident that this nightmare, like others in our history, will pass."

Promising a "compassionate society" based on "the civilized values of justice, freedom, and the rule of law" as well as Islamic principles, Reza appealed for support from all Iranians. "We must unite in love, equality and common purpose with no thought of bitterness, no sense of grievance and no recrimination."

Whether Reza will ever succeed in his quest of restoring the monarchy, let alone rally the opposition, seems a very hypothetical question at this point. Educated partly at Williams College in Massachusetts and trained in the United States as a fighter pilot, he has yet to finish his bachelor of arts degree. He is presently attending the American University in Cairo where he is a junior majoring in political science with economics as a minor.

While Egyptian President Anwar Sadat has given over the 200-acre Kubbeh Palace compound on the northeast edge of Cairo to the Pahlavi family, he has not bestowed his official blessing on Reza and avoided questions whether he will recognize him as the shah-in-exile of Iran.