Israel ordered its forces to observe a total cease-fire along the entire front in southern Lebanon yesterday. The unilateral move went into effect at 6 p.m. Israeli time.

The order was announced earlier in the day by Defense Minister Ezer Weizman and the timing coincided with Prime Minister Menachem Begin's meeting with President Carter in Washington.

Israeli radio reported last night that the truce was observed by Israeli troops, but gave no indications whether the Palestinians also were holding their fire.

The Palestinians failed to adopt an overall policy toward the cease-fire yesterday, Washington Post correspondent Jonathan Randal reported from Beirut. He said reports from the front indicated that all but small arms fire had ceased half an hour after the truce went into effect. Wire services reported from Israel that Israelis unleashed a fierce artillery barrage on Palestinian positions shortly before the truce went into effect.

[A PLO spokesman in Beirut said Israel's unilateral move "is not enough" and called for Israeli withdrawal. A spokesman for the radical Popular Front for the Liberation of Palestine called it "ridiculous" and vowed to wage "a relentless war of attrition."]

It is still unclear what Israeli forces will do if the Palestinians do not respect the unilateral cease-fire. In answer to a question, an Israeli military spokesman said that Israeli troops would evaluate each incident carefully and depending on the amount of Palestinian fire, the decision would be made whether to return the fire or not.

Finance Minister Simcha Ehrlich, the second-ranking figure in the ruling Likud Party, said last night, however, that Israeli troops would fire back if fired upon.

Earlier in the day the commander of U.N. forces in the Middle East, Finnish Gen. Ensio Silasvuo, met with the chief of staff of the Lebanese army in Beirut and told reporters afterward that if a cease-fire were not achieved, the conditions for getting U.N. troops into southern Lebanon would have to be re-examined.

It is widely assumed that Silasvuo will also try to press the Syrians, who are the predominant force in a 30,000 man inter-Arab force occupying Lebanon north of the Litani River, to try to enforce a cease-fire on the Palestinians.

It was not immediately clear tonight whether the unilateral truce would calm the concern surfacing among some countries whose troops might be used in the initial peace keeping force.

Austrian Chancellor Bruno Kreisky said earlier in the day that he would not commit troops before a cease-fire on the ground was fixed.

Although it was reported last night that some 250 Swedish soldiers were on six-hour alert for possible Lebanon duty, there were also reports reaching here earlier that the Swedish Cabinet had postponed a meeting to decide whether to give final approval.

Both of these countries have forces already on UN duty in the Golan Heights and the Sinai. A few hundred of them, sources here say, are included in the UN contingency plan designed to speed an initial force into the area - before the main 4,000-man force can be assembled - to help preserve the momentum set in motion by the UN Security Council resolution last Sunday.

Canada, which provides logistics and communications on the Golan Heights, had also indicated a general willingness to help in Southern Lebanon. As one official put it prior to the cease-fire announcement however, "The security of our troops is a real concern.We don't want to send them into a meat grinder."

"If you don't have a situation where both sides agree to a cease-fireand a UN presence," a Western military attache said, "the UN does not work. Unless the Palestinians agree to a cease-fire and the UN resolution, you're not going to find too many countries rushing in with troops."

Lebanon has proved to be a dangerous place over the past year for unarmed UN observers. Several have been robbed and roughed up by both the Palestinians and their enemies, the Lebanese Christian military. The new peace-keeping force would be armed.

Earlier yesterday Israeli Foreign Ministry officials said that the cease-fire problem could become a major obstacle to working out the Israeli withdrawal called for in the UN resolution.

Before the announcement, Israel reported that there were periods of artillery exchanges and that a few Soviet-built satyusha rockets landed in the northeastern part of the country.

Actually the language of the Security Council resolution does not call for a cease-fire and it applies only to Israel, something which angers Jerusalem and reflects what Israelis view as a UN double standard.

It called for Israel to "immediately cease its military action against Lebanese integrity and withdraw forthwith its forces from all Lebanese territory."

Israeli officials say they interpret the cessation of military activity to be the equivalent of a cease-fire and as recently as Monday night, the defense minister was saying that Israel was ready for an immediate cease-fire but that it had to be declared by two sides.

Weizman told some members of the ruling Likud Party faction last night that, in his opinion, the sooner the Israeli army could get out of southern Lebanon the better so that Israel could go back to the main problem of exploring the peace initiative of Egyptian President Anwar Sadat, Israeli television reported.

He said that the Israeli command had been taken by surprise by the suddenness of the U.N. resolution calling for Israeli withdrawal but he thought thatIsrael should make the U.N. peace-keeping force could do the best job it could.