Israel used cluster bombs extensively against civilians in Lebanon and "flagrantly violated" agreements with the United States in doing so, according to an American international-law specialist who spent a month in Beirut obtaining affidavits from doctors, interviewing victims and gathering bomb parts.

Franklin Lamb, who holds a doctorate in international law, said he is distilling the evidence into a report and hopes to persuade the U.S. government to make permanent President Reagan's temporary ban on shipment of cluster bombs to Israel.

Cluster bombs, each consisting of hundreds of "bomblets," can be dropped by planes or fired from artillery. The bomblets are produced in three shapes: round, about the size of tennis balls; cylindrical, like orange juice cans, and in the configuration of a butterfly.

Each bomblet explodes into broad showers of steel pellets or shards, slicing through skin and pulverizing muscle and bone with a deadly spinning action. A U.S. Marine was killed at the Beirut airport two weeks ago by a bomblet the United States had sent to Israel.

"There is credible, substantial, probative evidence that Israel has flagrantly violated existing laws," Lamb said of the material he gathered in Beirut from late July through late August. "Next comes the political judgment of what to do about it."

Lamb said he thinks the president and Congress should declare Israel guilty of violating U.S. restrictions on use of U.S. weapons and announce that the existing suspension on cluster-bomb exports will be permanent.

"Cluster bombs have become the napalm of the Middle East," Lamb said.

Since Israel now makes its own cluster bombs, Lamb said, a U.S. ban would not harm Israeli security. But the absence of a permanent ban "only ends up hurting the American people, who are blamed for their use in Lebanon," he said.

Lamb, 38, a former college lecturer active in civil rights causes in the 1960s, said that no Arab organization participated editorially in his report but that the Arab Information Office, which had urged him to investigate conditions in Lebanon, had paid for his airplane ticket.

Doctors in 19 hospitals and clinics in West Beirut signed affidavits swearing they had treated cluster-bomb victims, including the elderly and children, he said. The draft of his report, to be released this month, also says:

* Unexploded or parts of four different types of U.S.-made cluster bombs -- designated CBU Mk20, CBU 58, M42 and M43E1 -- were found in refugee camps, including Shatila and Sabra in Beirut, and 14 locations in the heart of West Beirut, including the Hamra central post office.

* Lebanese neighborhood groups distributed posters depicting various cluster bombs and warned against touching unexploded remnants.

The Washington Post showed a photograph of the warning posters to Maj. Gen. Menachem Meron, Israel's military attache here. He confirmed Israel's use of the types of cluster bombs depicted against military targets in Lebanon.

Cluster bombs "were used only against military targets," Meron said, adding that "we regarded the PLO Palestine Liberation Organization armed units as military targets."

Using that rationale, Israel could argue that any cluster bombs dropped on refugee camps were aimed at PLO combatants, not civilians.

On Cable News Network's "Newsmaker" program aired Oct. 2, Meron said, "We used cluster bombs in Beirut according to the agreement that we had with the United States. . . . It's very difficult to distinguish between those that we received from the United States and those that we produced ourselves."

U.S.-Israeli agreements restrict use of cluster bombs to self-defense, according to U.S. officials. The White House announced July 19 that "there will be no shipment of artillery projectiles or other cluster-bomb-unit related materials" until the administration determines whether Israel violated conditions imposed on their use.