President Anwar Sadat slipped unobtrusively out of Egypt today on his way to Washington for the signature of a peace treaty with Israel.

Far from treating it as a historic event to be celebrated, the Egyptians kept Sadat's departure as quiet as possible, apparently for security reasons.

There was no live television coverage and no announcement was made until after the presidential plane had taken off from Cairo airport. Even officials who were traveling with Sadat said they were kept in doubt about his schedule until this morning, and a scheduled overnight stop in Madrid was canceled at the last minute.

Sadat arrived at Andrews Air Force Base outside Washington tonight.

The official Middle East News Agency quoted Sadat as saying he was "very happy" to be embarking on his historic journey, but he refrained from saying anything of substance, as he has since he gave President Carter his approval of the final compromises on the treaty two weeks ago.

The feeling here about Sadat's trip seems to have been expressed by a newspaper editorialist, who said it was the conclusion of a quest for peace that began with Sadat's voyage to Jerusalem, a trip "tantamount to a trip to the end of the world or another planet," exciting but fraught with danger.

The low-key approach today is a reflection of that uncertainty, as anticipation of peace and relief that it has at last been achieved are mixed with apprehension about the future. The foreign minister of Syria, Abdel Halim Khaddam, appears wide of the mark in predicting that Sadat will be overthrown because of the treaty. The streets of Cairo are quiet on a lovely spring weekend dominated by talk of soccer and the International Trade Fair. But there is no atmosphere of celebration.

Egypt is, predictably, the target of furious and escalating criticism from the other Arabs.

Strong denunciations of Sadat are expected at the semiannual meeting of the Arab League which opens today in Somalia-it has always been held in Cairo in the past, but was moved in response to Arab outrage over the Camp David agreements of last September-and at a meeting of Arab foreign and economic ministers opening Tuesday in Baghdad, Iraq.

Coincidentally, the Cairo weekly magazine October said Egypt will announce "that it is freezing its membership in the (Arab) League as well as its activity in all organizations attached to the League."

"Egypt will announce its withdrawal" from the League afterward, said the magazine, which has close connections with Sadat. The reported Egyptian decision appeared aimed at obviating expulsion, an action likely to be taken at the Baghdad meeting.

Several hard-line Arab countries long ago broke diplomatic relations with Egypt, and reports from Beirut today say the Palestine Liberation Organization has decided to close its office here.

Sadat, who in the past has blasted his Arab critics in insulting terms, has refrained from doing so this time, but he has also moved ahead undeterred by what are seen here as rhetorical posturings from other Arab capitals.

Sadat will be away more than a week. Upon his return he will address the People's Assembly, or parliament, which will then be asked to ratify the treaty. An overwhelming vote in favor is assured.

Wire services also reported these developments:

Soviet Foreign Minister Andrei Gromyko arrived in Damascus yesterday for talks with Syrian President Hafez Assad. The talks are expected to focus on the U.S. sponsored Egypt-Israel peace treaty, opposed by both Syria and the Soviet Union. The Kremlin is Syria's chief source of arms and economic support.

In Cairo, the semiofficial newspaper Al Ahram quoted Egyptian military sources saying Israel will start pulling military equipment out of the Sinai peninsula Sunday as a prelude to complete evacuation under terms of its peace treaty with Egypt.