President Reagan formally notified Congress today that U.S. Marines have been involved in "sporadic fighting" this week in Lebanon, but he avoided a congressional review of their continuing presence there.

Declining to invoke a provision in the War Powers Act under which Congress could try to force him to withdraw the Marines from the multinational force in Lebanon within 60 days, Reagan declared in a letter to congressional leaders:

"I believe that the continued presence of these U.S. forces in Lebanon is essential to the objective of helping to restore the territorial integrity, sovereignty, and political independence of Lebanon."

Reagan said "it is still not possible to predict" how long the Marines will remain in Lebanon.

The president sent the letter after a day of conferring by telephone from his mountaintop California ranch with key foreign policy advisers here and in Washington.

White House spokesman Larry Speakes said Reagan directed his staff to provide him with frequent, regular reports on the military situation in Lebanon and also directed his special Middle East envoy, Robert C. McFarlane, to "vigorously pursue" efforts to help reconcile warring Lebanese factions.

Reagan also asked Israel to delay for another 72 hours the planned withdrawal of its occupying forces from the Chouf Mountains region near Beirut to positions in southern Lebanon, officials said. The Israeli government agreed with unanticipated swiftness to Reagan's request, made in a letter to Israeli Prime Minister Menachem Begin, according to the officials.

The delay would "provide an opportunity for diplomacy to work," Speakes said, as McFarlane continues his attempt to win agreement from the warring Lebanese factions to allow the Lebanese government army to move without opposition into positions evacuated by the Israelis.

"It is our goal to assure the Lebanese government can restore their authority over their sovereign territory and that there be a minimum of disruption," Speakes said.

Reagan's letter to House Speaker Thomas P. O'Neill Jr. (D-Mass.) and Senate President Pro Tem Strom Thurmond (R-S.C.) detailed the "sporadic fighting" that erupted Sunday in Beirut between the Lebanese army and various religious factions, in which the Marines have come under frequent attack.

The letter said a "cease-fire" came into effect late today in Beirut. "Diplomatic efforts are under way to extend this cease-fire," Reagan said. "In the meantime, U.S. forces will be prepared to exercise their right of self-defense should such attacks recur."

Speakes said a formal cease-fire was not in effect but a "ceasing of military activities" had taken hold.

He said the Marines suffered no casualties today when their positions came under small arms, rocket-propelled grenade and mortar fire. The Marines responded with rifle and machine gun fire and later with illumination rockets until the fighting stopped, according to Speakes.

"What you've got here--it's not a war with a front," said one senior administration official. "It's action throughout Beirut. These are incidents."

Telling reporters that this should not trigger a congressional review of the deployment of Marines--along with French, Italian and British troops--in the multinational force in Lebanon, Speakes said, "If we were conducting combat operations it would be different. But we are not. The role there is as a peace-keeping force."

"These Marines are in stationary positions," the senior official said, adding that it would be combat only if the Marines left their positions for "sweeps" elsewhere in Lebanon. Asked if it was the administration's position that the Marines' retaliatory fire did not constitute a combat operation, the official said, "That's right."

Pentagon officials said that if the Marines were in a combat role, they could patrol around their encampments to clean out threatening forces. But the officials said the president has ordered the Marines to act as a military "presence" and fire only in self-defense.

"If we pull out, there will be a real bloodbath. We're really stuck," said one Pentagon official. "But taking these casualties makes it tough."

Sen. Charles McC. Mathias Jr. (R-Md.), an author of the War Powers Act and member of the Senate Foreign Relations Committee, joined a handful of other lawmakers in urging Reagan to seek congressional authorization if he intends to keep the Marines in Lebanon.

In response to similar proposals Monday from House Foreign Affairs Committee Chairman Clement J. Zablocki (D-Wis.) and Sen. John Glenn (D-Ohio), Senate Armed Services Committee Chairman John G. Tower (R-Tex.) said such a provision of the War Powers Act requiring congressional authorization "could conceivably be triggered by what happened" in Lebanon.

Former vice president Walter F. Mondale, a contender with Glenn for the Democratic presidential nomination, called for a reevaluation of the Marines' presence in Lebanon and suggested the War Powers Act as a "vehicle" for the debate.

As passed by Congress in 1973 in response to unauthorized presidential commitment of U.S. troops in Vietnam, the War Powers Act sets up procedures under which the president must consult Congress before sending troops abroad and report back to Congress thereafter.

One of the act's key provisions bars the president from keeping military forces in hostile situations abroad for more than 60 days unless Congress has declared war, authorized the specific troop deployment or extended the time period for another 30 days.

Another would permit Congress, in a so-called legislative veto by concurrent resolution of both houses, to force the withdrawal of troops that have been committed to hostilities abroad without congressional authorization within the 60-day period.

Such legislative vetoes were declared unconstitutional by the Supreme Court last June. But most congressional authorities contend that the rest of the War Powers Act, including the 60-day period for congressional authorization, remains valid. The Reagan administration also indicated in congressional testimony shortly after the Supreme Court ruling that it intends to comply with the War Powers Act, minus its legislative veto provisions.

The administration complied with the act's reporting provisions when it committed U.S. troops to Lebanon last year, but it did so in a way that did not trigger the 60-day authorization period.

Congressional approval is required when U.S. forces are introduced "into hostilities or into situations where imminent involvement in hostilities is clearly indicated by the circumstances." In informing Congress that it was sending Marines to Lebanon last year, the administration said it had "no intention or expectation that U.S. armed forces will become involved in hostilities."

But the law also requires that the president report to Congress within 48 hours of U.S. troop involvement in any hostilities. The law is written in such a way that Congress can invoke the authorization process even if the president declines to do so.

Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.) has not yet said whether he believes the 60-day provision should be invoked for Lebanon. But an aide said he has indicated that the committee will want full briefings on the issue after Congress returns from its summer recess Sept. 12. Hearings also are expected in the House.

Mathias said he believes it may be necessary for Congress to return earlier than Sept. 12 if hostilities continue and Reagan does not seek congressional authorization. With most other lawmakers on vacation, it was not clear how widely his views are shared.

The Beirut fighting interrupted what was expected to be a calm week while Reagan vacationed at his ranch 30 miles from here in the Santa Ynez Mountains. Top foreign policy aides conferred by secure telephones with Reagan from the Santa Barbara Biltmore Hotel here and from the White House in Washington.

At the White House, Vice President Bush today chaired another meeting of a "special situation group" on the Beirut crisis, which included Defense Secretary Caspar W. Weinberger and Secretary of State George P. Shultz. The group had ordered a review of the War Powers Resolution on Monday and reported to Reagan today.