President Anwar Sadat told Saudi Arabia and its Persian Gulf neighbors today that the United States is their only real defense against the threat of Islamic Revolution from Iran or military pressure from the Soviet Union.

He also announced that, in protest against the Soviet invasion of Afghanistan, he had ordered a reduction in diplomatic personnel at the Soviet Embassy in Cairo from 50 to seven and the expulsion of remaining Soviet technical experts in Egypt.

In another development, Reuter reported from Cairo that Egypt had asked Israel to postpone sending an advance team of Israeli diplomats to the Egyptian capital until Feb. 15. Israeli foreign ministry officials expressed surprise at the move, saying no explanation was given, according to Washington Post correspondent William Clairborne in Jerusalem.

[However, Egyptian Foreign Ministry officials reportedly said the postponement was in line with Sadat's decision to consider Feb. 15 the date for starting the full normalization of relations between the two countries. The officials said ambassador would be exchanged Feb. 26, as stipulated in the peace treaty signed by the two countries last March.]

Sadat's warning to the Saudis and other Gulf Arabs amounted to a call for more cooperation from Arab leaders reluctant to openly accept President Carter's new doctrine for Middle East security in which Washington pledges to resist any military intervention against its vital interests in the Gulf oil states.

It came in a 2 1/2-hour speech to the Egyptian People's Assembly devoted mainly to defense of Egyptian peacemaking with Israel and the increasingly pro-Western orientation of Sadat's foreign policy, which he underscored by announcing the Soveit Embassy staff cutback and decision to send home Soviet civilian advisers.

Although the number of advisors is not public, Western intelligence sources estimate several hundred still work in factories or agricultural projects. This is a far cry from the roughly 20,000 Soviets who were here at the height of Moscow's influence in 1971, giving Sadat's order more political than practical importance.

Sadat, speaking one day after Egypt opened its land border with Israel for the first time in three decades, strongly criticized his fellow Arabs, who oppose Egypitan participation in the Camp David accords, the peach treaty with Israel and the negotiations to set up Palestinian autonomy in Gaza and the West Bank.

As he has in the past, he singled out Saudi Arabia, which he said is leading the opposition to his peace policies. In remarks calculated to irritate the Saudis, he characterized the recent take-over the Grand Mosque in Mecca as a political uprising against the Saudi royal family.

"Saudi Arabia and the Saudi royal family are in difficulty," he said. "They are in a crisis."

The criticism directed against Saudi Arabia coincided with a visit here by Sol Linowitz, Carter's special Middle East Envoy, who also is scheduled to see the Saudi leadership in Riyadh next Friday to encourage Saudi cooperation in the autonomy talks.

Observers, noting the coincidence, predicted it would not facilitate Linowitz' discussions with the Saudi royal family.

In his speech, Sadat also said Saudi Arabia had made a friendly gesture to the Soviet Union by allowing Soviet military planes to fly through Saudi air space with military supplies for the Marxist, regime in South Yemen. This, he added, amounted to "playing off the United States against the Soviet Union."

"All the leaders of the Persian Gulf should know very well that the source of their defense is in the United States," he went on. "Egypt has a commitment and I want the Arab leaders to know that if any of the Gulf states or any Arab state faces a threat from the Soviet Union or from Iran, I will give full facilities to the United States to face it, without any need to ask for them."

This was a reiteration of an Egyptian offer announced early this month by Defense Minister Kamal Hassan Ali for "facilities" for help from the Gulf. At the time, Ali said U.S. and Egyptian forces had participated in joint exercises to familarize Americans with the Egyptian facilities.

In effect, U.S. sources said, this meant U.S. crews used an Egyptian air field for reconnaissance including flights by AWAC airborn radar centers. Knowledgeable sources said the flights also passed over Saudi territory on the other side of the Red Sea from southern Egypt, including Saudi cooperation as well.

A Saudi spokesman denied yesterday that Saudi Arabia had participated with Egypt in any joint excerises, underlining Saudi reluctance to be seen as part of a U.S.-sponsored defense framework as long as Washington remain unresolved.

It was this priority to Arab solidarity against Israel that Sadat seemed to be attaching most directly in his remarks. The real danger lies elsewhere in Soviet designs, he suggested. After detailing several instances when he said the Soviet Union let down Eygpt, carried out "faithfully and with honor" its Camp David commitments to withdraw from two-thirds of the Sinai by last Friday.

His remarks also were aimed at the Islamic foreign ministers' conference in Islamabad, Pakistan, where Egypt's Arab opponents are urging that condemination of Israeli occupation of Arab land be attached to any condemination of Soviet intervention in Afghanistan.

The preoccupation with Israel was echoed by King Hussein of Jordon, whom Linowitz saw Saturday in London at the outset of his Middle East swing. Hussein and his prime minister, the Sherif Abdul Hamid Sharaf, repeatedly have said that for Arabs the real threat comes from Israel, not the Soviet Union.

U.S. sources said Linowitz' discussions with Hussein were cordial but did nothing to change the monarch's opposition to the Palestinian autonomy talks. But they added that Hussein had accepted an invitation relayed by Linowitz to visit the White House for talks with Carter.No date has been mentioned for the visit, however.