Senate Foreign Relations Chairman Charles Percy (R-Ill.) said in a hearing last Tuesday that U.S. intelligence officials had confirmed to him that 10,000 civilians had been killed in the Israeli invasion of Lebanon.

Two days later, State Department spokesman Dean Fischer told reporters that "there are no reliable figures available at this time." On Friday, congressional sources said privately that Central Intelligence Agency officials had told them in briefings that the figure was between 5,000 and 10,000.

Meanwhile, the Israeli government issued a new report putting civilian dead at 600, while Lebanese officials in Beirut said their count showed 18,000 killed.

And so the thicket of claims and counterclaims over the number of civilian dead grew more dense last week with the participants in the conflict and the United States compounding the confusion and uncertainty in what has emerged as the key issue in the debate over whether Israel's invasion was justified.

Widely disparate figures have been presented for civilian deaths, as well as for civilian wounded and refugees, and Israel and its opponents have accused each other of grossly distorting the body count for political purposes.

The numbers game thus has become the main ammunition in the other war that Israel and its opponents are waging--the propaganda war that Israeli Prime Minister Menachem Begin has called "a battle over the truth."

The United States has added to the fog by issuing conflicting accounts of the casualties. Percy said intelligence officials labeled as accurate an estimate of 10,000 civilian deaths that he first received from the Lebanese ambassador to the United Nations. Percy added that the "unconfirmed" estimate may be as high as 14,000.

Percy's executive assistant, Scott Cohen, said Friday that Walter J. Stoessel Jr., while acting secretary of state, had told Percy that the government "accepted" that figure, which has been used publicly by other senators, including Paul Tsongas (D-Mass.) and Claiborne Pell (D-R.I.).

Sources in the House of Representatives say the CIA gave them different figures--between 5,000 and 10,000. They said confusion may have arisen because the 10,000 figure included a CIA estimate that as many as 3,000 of those killed were combatants. But there appeared to be no way to verify independently the CIA's figures, nor determine on what they are based. Katherine Hall, a CIA spokeswoman, said yesterday the agency had no comment on the matter.

Sen. Patrick J. Leahy (D-Vt.), a member of the Senate Select Intelligence Committee, said yesterday that much of what he had been told in intelligence briefings had been "nothing more than a rehash of what was in the newspapers . . . . I've heard numbers all over the place depending on who's doing the briefing."

Many firsthand observers of the scene in southern Lebanon agree with Fischer that an accurate estimate is not possible. The mass movement of civilian populations fleeing the war, the possibility that large numbers of civilians who sought shelter in basements of buildings were buried alive in bombing and shelling attacks, the Moslem custom of burying the dead within 24 hours of death--all have been cited as reasons why no accurate total can be determined.

"You have to picture the scene of the entire population of South Lebanon fleeing back and forth across the landscape trying to get out of the battle zone," said Dr. Christopher Giannou, a Canadian surgeon and Palestinian supporter who worked in Sidon before his arrest by the Israelis last month. But Giannou, who later was released without charge, said at a press conference here last Friday that he was convinced from his own observation that the death toll in Sidon alone far exceeded Israel's claim for the entire operation.

Israel has buried large numbers of the dead in mass graves and spread lime over decomposing corpses. Israeli officials say those actions were taken for health reasons, but opponents say the burials also have been used to conceal the death toll.

American supporters of the Israeli military operation say that no matter how high the civilian losses, the invasion is justifiable because the Israeli Army has taken massive and unusual precautions--such as dropping leaflets on cities warning of attacks and allowing time for evacuations--to protect civilian lives and because the continuing presence of the Palestine Liberation Organization in Lebanon had led to civil strife causing the deaths of between 70,000 and 100,000 civilians during the past decade. They also blame the PLO for civilian deaths, asserting that PLO fighters used schools, hospitals and other civilian institutions for shelter from Israeli assaults.

"Even if the 10,000 figure is correct, as horrible and poignant as that would be, it has to be seen in the context of the terrible figures of the past," said Hyman Bookbinder, Washington representative for the American Jewish Committee. "Only history will judge--if this operation puts a final end to the killing in Lebanon, then it may prove worthwhile."

Israel's supporters concede that early estimates of high losses cost Israel some popular support and caused some longtime American Jewish supporters of Israel to oppose the invasion. Even more important, high civilian losses challenge the widely held belief in this country that Israel is a special nation with a moral mission, deserving of its special relationship with the United States and the massive military and economic aid it receives.

"For the Israelis what's at stake is their self-image both at home and abroad as a decent people," said Seymour Martin Lipset, a noted Jewish sociologist who has opposed the invasion.

Leahy said many congressmen, including himself, would weigh the numbers of civilian deaths in deciding their future attitude toward Israel. "It will have a direct bearing on a lot of foreign policy issues," he said.

American supporters of the Palestinian cause have used their own estimates of civilian deaths to challenge Israel's image and credibility.

"The Israelis have deliberately and consciously lied about the casualty figures," said James Zogby, executive director of the Arab-American Anti-Discrimination Committee in Washington. "I don't want to dicker over numbers. Something awful has happened and we all ought to be ashamed of it . . . , but we are confronted with a Goebbels-type big-lie campaign trying to tell us it didn't happen at all."

Zogby said he based his own estimate of 14,000 dead on figures provided by church groups, doctors and eyewitnesses in Lebanon, along with a report issued late last month in Rome by Caritas, the Vatican-related relief agency.

Whatever the correct total, Arab-American groups have used the death toll to mount what both sides agree is one of the most effective anti-Israeli lobbying drives ever seen in this country. There have been newspaper advertisements accusing the Israelis of genocide against the Palestinians and Lebanese, radio commercials and nationwide tours funded by a variety of pro-Arab groups, and even a slick, 24-page brochure of photographs from the war issued by the Arab Information Center in New York, the registered U.S. agent of the Arab League.

Some of the ads have been questioned. One, sponsored by a Los Angeles-based group calling itself Concerned Americans for Peace, that appeared in The New York Times last Sunday cited "news reports" placing the number of dead and wounded at 40,000. The same ad, run in The Washington Post on the same day, put the figure at 26,000.

The ad was criticized by six relief organizations whose names and addresses were printed without authorization. The listed telephone for Concerned Americans has been disconnected, and the Bernard Hodes Agency, which placed the ad in newspapers around the country, has said it has been unable to contact the group.

Israel has charged that ad campaigns built around casualty figures are part of what Israeli Ambassador Moshe Arens has labeled "a campaign of vituperation and slander of unprecedented proportions." Israeli officials here have charged that Arab interests have spent more than a million dollars on the effort.

"It's a dirty tricks campaign," said Israeli Embassy spokesman Nachman Shai. "If you throw out a number--10,000, 20,000, whatever--how can I prove it is wrong? No one can say for sure. But the operation was carried out very carefully and with tremendous attention to civilian life even at great risk to our own soldiers."

Pro-Israeli groups here have mounted a countercampaign using newspaper ads and heavy lobbying in Washington.

Arab-American opponents scoff at the claim that the long-established Israeli lobby here is losing the propaganda war here because it is being outspent.

"They haven't been outlobbied by the Arabs, they were outlobbied by the truth," said former senator James Abourezk, a Lebanese American. "What's worrying them is that people got turned off by what they did, and so now they're trying to cover it up."

Abourezk said he believed the Israeli invasion would prove "a dramatic turning point" in U.S. support for Israel.

"The public can smell that something's wrong," he said. "It will take elected politicians two or three years to catch up, just as it did with Vietnam, but they will."

Many Jewish Americans are enraged by Arab use of World War II-style words such as "holocaust" and "genocide" to describe what is happening to civilians in Lebanon. "They're murdering the language in an obscene effort to compare today's tragedy with a unique and unprecedented tragedy of 40 years ago," Bookbinder said.

But even some of Israel's most ardent supporters agree that the war has made them uncomfortable in a way they never were before. Rabbi David Saperstein, Washington representative of the Union of American Hebrew Congregations, stressed his belief that the Israelis, having decided to go into Lebanon, did all they could to protect civilians.

But, he added, "How can you not be troubled? The higher it the death toll goes, the more troublesome it is for American support for Israel and for the Israeli people. When you balance the loss of human life and the possible erosion of support, Israel has to ask, was it worth it?"