Israeli officials expressed concern today that the abrupt change in American secretaries of state could lead to a rapid shift in U.S. policy that will make it more difficult for Israel to realize its far-reaching objectives in Lebanon.

The doubts reflected a conviction here that Alexander M. Haig Jr., who resigned as secretary yesterday, was Israel's principal backer in the Reagan administration during the invasion of Lebanon and that Secretary-designate George P. Shultz is less likely to accept Israel's view of the crisis.

The officials, speaking anonymously, said Haig's resignation means Israel must move quickly to accomplish its goals before changes in U.S. policy take effect, particularly because Shultz, they believe, will be a powerful ally of Secretary of Defense Caspar Weinberger in questioning the operation in Lebanon.

No one defined what "moving quickly" means. Even after a U.S.-engineered cease-fire took effect last night in Lebanon, Israeli Defense Minister Ariel Sharon refused to rule out an all-out ground assault on West Beirut's Palestinian strongholds.

Despite the cease-fire, Israeli warplanes knocked out a battery of SA6 antiaircraft missiles that had been moved into Lebanon's Bekaa Valley from the Syrian border, the Israeli command announced. It was the only military action reported by Israel during the day.

In an apparent effort to head off more Israeli strikes, the Reagan administration today sent a message assuring the Israeli government that Haig's resignation does not mean a fundamental change in Washington's Middle East policy, the state-run Israeli radio reported, without naming a source.

At the same time, Eliahu Ben-Elissar, chairman of the parliament's security and foreign affairs committee, said that Israeli plans to use diplomacy to reach its goals in Beirut.

"It is right to say that we are now in the diplomatic stage," he said, pointing out that reports from Beirut indicated the Lebanese capital was quiet throughout the day.

[As concern grew about the human cost of the war, about 2,500 people demonstrated in Tel Aviv in protest aganst the conflict, The Associated Press reported. Demonstrators waved posters with slogans such as "Stop Israeli aggression in Lebanon" and "Stop the bloodshed now." Speakers called for the resignation of Sharon and the immediate withdrawal of Israeli troops from Lebanon.]

Sharon and Prime Minister Menachem Begin did not comment today on why they silenced Israeli guns and warplanes after yesterday's relentless bombardment of Beirut.

Their spokesmen said they were resting on the Sabbath and preparing for Sunday's meeting of the Cabinet.

Sharon hinted twice yesterday, however, that he was unhappy with pressures for a cease-fire. He called on Israelis to remain united and to ignore "political provocations, internal or external, that exist and that may threaten our military and political achievements."

Sharon continued: "The Palestine Liberation Organization is on its way to complete destruction in Lebanon and in Beirut if we do not leave them alone now."

Sharon's comments underlined the chief goal of the Lebanon operation -- disarming the PLO and eliminating its leaders and the core of its fighters by killing them, capturing them or banishing them to a country that does not border Israel. With this in mind, Begin's government strongly rejected the French U.N. Security Council resolution offered yesterday, which would have called on PLO guerrillas to pull back into refugee camps without disarming.

The Israeli Foreign Ministry expressed its "deep satisfaction" at the U.S. veto of the French resolution, but Israeli sources nevertheless voiced fears that with the cease-fire in place, U.S. special envoy Philip C. Habib could make compromise suggestions that would fall short of Israel's insistence on eliminating the PLO as an organization.

A former Israeli ambassador to the United States, Simcha Dinitz, said the greatest chance of such a move will come during the next few days. Lower-ranking U.S. officials could take advantage of a leadership vacuum before Shultz takes office, working out a solution through Habib that would leave some guerrillas armed in their camps, he said.

Demonstrating its concern, the Israeli government reminded Habib today that Israel still insists that all guerrillas be disarmed as part of any settlement, the government-run Israeli radio reported.

Before the announcement of Haig's resignation and yesterday's reports of pressures from Washington, Begin's Cabinet had expressed the conviction that the Reagan administration was backing the Lebanon operation.

Several ministers told Israeli journalists that Begin returned from Washington on Wednesday with the belief that U.S. protests would be mild about any Israeli actions short of a ground assault on West Beirut -- would be mild.

Against that background, Sharon ordered extensive bombing and shelling of West Beirut. Israeli officials explained the heavy bombardment as an attempt to force the PLO leadership to surrender.

The possibility that Washington now has drawn a line, however, has led Israeli officials to worry whether the Israeli objective -- extermination of the PLO as a political or military threat -- is endorsed as enthusiastically in Washington as they had been led to believe.

Sunday's Cabinet meeting, it was pointed out, will help determine whether the Israeli government -- and particularly the hard-charging Sharon -- is willing to slow the momentum of its increasingly damaging attacks on the guerrillas long enough to find out.