Beirut — Within two days of the departure of United States Secretary of State George Shultz from the Middle East, the euphoria of peace had plummeted to predictions of possible war. And a number of sudden changes have raised new challenges to Lebanese security:
* Syrian forces shelled Israelis in the tense eastern Bekaa Valley Tuesday.
* Western diplomats confirmed that the Palestine Liberation Organization had sent reinforcements into Lebanon from Syria.
* More than 80 dependents of Russian diplomats in Lebanon were evacuated Monday, with more expected to leave Tuesday.
* Tension remained high in the Shouf mountains east of Beirut, where rival Christian and Druze militias have been fighting sporadically for the past six days.
The combination of developments served to heighten apprehension about a conflict between the Israelis and Syrian-Palestinian forces played out on Lebanese soil.
The crumbling military situation also comes at a time of political problems connected with the agreement worked out by Mr. Shultz during his two-week shuttle.
There were widespread reports in the Beirut press Tuesday about Lebanese concern over Israeli demands in the so-called ''clarifications.'' Lebanese officials were quoted as accusing Israel of trying to obtain concessions through the clarifications that it did not gain during the Shultz tour.
Radio Israel suggested Tuesday that the difference in positions was significant enough for Israeli leaders to ask for a resumption in the tripartite talks that preceded the Shultz tour. But Lebanese Prime Minister Shafik Wazzan responded: ''We do not intend through clarifications to revise what we had already committed ourselves to.''
The Israeli-Lebanese agreement on a formula for the withdrawal of foreign forces from Lebanon has also come under increasing attack from the Syrians, who have an estimated 40,000 troops in Lebanon. The state-controlled newspaper Al Baath editorialized: ''Our decision in rejecting the Shultz plan is like our rejection of the Reagan plan and the Camp David agreement. . . . We are also capable of freezing this Lebanese-Israeli agreement because we shall not allow anyone to use Lebanon and turn it into an Israeli-American protectorate threatening our security.''
In a more ominous note, it added: ''We have prepared ourselves for all eventualities, and no force can undercut our will to wrest our rights from the mortal danger threatening our people.''
The angry words were backed up by action when the Syrians fired six shells at the Israelis in the most serious incident since the fighting during the Israeli invasion last summer. Although it was a limited action, it did reflect the raw tension between the two arch rivals and the danger that events on the ground may overtake the US-orchestrated peace agreement.
The dispatch of additional PLO troops into the Bekaa also did not bode well for eventual implementation of the plan.
A PLO source in Damascus claimed the move was in preparation for an Israeli attack, which the Syrian press has been predicting for weeks.
At a press conference Monday, PLO spokesman Abdel Mohsin Abu Maizer said the guerrilla movement ''condemns and rejects'' the accord. And Bassam Abu Sharif of the Popular Front for the Liberation of Palestine faction said the PFLP backed Syria's position because the agreement ''is an attempt to force Lebanon to kneel under the pressure of the Israeli conditions.''
The fighting among rival Lebanese factions also contributes to the atmosphere of conflict, since the Druze have traditionally been supported by the Syrians, and the Christian Phalange forces have been backed by Israel. It is, in effect, a microcosm of the inter-nation sparring.
Officials of the multinational peacekeeping force also claim that at least some of the shells that spilled over into Beirut during factional fighting last weekend appeared to have been fired from behind Syrian lines.
But the event that has had the biggest impact psychologically was the sudden evacuation of families of Russian diplomats. An embassy official claimed the children were returning home for summer vacation, although school break is still a month away. East-bloc diplomats later said the move, unprecedented during peace time, was due to the deteriorating security situation in Lebanon.
Lebanese President Amin Gemayel was sufficiently angered to summon Soviet Ambassador Alexander Soldatov Tuesday to demand an explanation for the evacuation, as well as for a current Soviet news media campaign that is highly critical of the Lebanese-Israeli agreement.
The evacuation follows the escalation of arms shipments from the Soviet Union to Syria, where US intelligence sources claim there are now 5,000 Russian troops and advisers. Under the terms of a treaty of friendship and cooperation, the Soviet Union has made it clear that it will intervene in the case of an attack on Syria.
The reaction among the Lebanese to the evacuation was typified by a Lebanese who said: ''It sure makes you feel like they know something we don't.''