Israel added a new dimension to the Middle East arms race on Tuesday by using eight F15 fighters as long-range bombers in a 1,500-mile strike against Palestine Liberation Organization headquarters in Tunisia, Defense Department officials said yesterday.

Israel since 1978 has opposed selling F15s to Saudi Arabia, partly on grounds that the plane would not remain a defensive fighter but would be transformed into an offensive bomber threatening the Jewish state, especially if the United States supplied bomb racks with the plane.

Israeli opposition to the bomb racks became so intense that on Oct. 27, 1980, then-President Jimmy Carter said "We will not agree to provide offensive capabilities for the planes that might be used against Israel. And that obviously includes bomb racks."

Pentagon officials said Israeli employment of the F15 as a long-range bomber will provide an additional argument for members of Congress opposed to the sale of either the F16 or F20 fighter bomber to Jordan. Opponents can now argue, the officials said, that these planes also could be used for long-range bombing, posing threats to Israel as well as other countries.

Former senator James G. Abourezk (D-S.D.), chairman of the Arab-American Anti-Discrimination Committee and a spokesman for Arab causes, said yesterday that the Israeli strike "upsets the arms balance in the Middle East and touches off a new arms race because Arab countries will seek weapons to protect themselves against such long-distance strikes."

Israel has 56 F15s, all capable of being equipped for long-distance bombing raids by adding fuel tanks and bomb racks known as MERs, for multiple ejection racks. The Air Force sent MERs to Israel in 1978 for the F15, and Israel is believed to have built others since. With Israeli-made conformal fuel tanks, which fit along the plane's fuselage, the F15 can carry a small load of bombs to a target 900 miles away and return to base without refueling, sources said. With wing tanks added to the conformal tank, the F15 could go even farther.

Israel used a mix of heavy conventional bombs and light cluster bombs on the raid against the PLO headquarters at Borj Cedria, 21 miles south of Tunis, sources said.

The big bombs knock down buildings, while the cluster bombs shower shrapnel to kill people and destroy machinery. Pentagon officials said they assumed, but did not know with certainty, that Israel used American-made tankers to refuel the F15s in mid-air and perhaps sent U.S.-built E2C Hawkeye command and control aircraft to guide the bombers and watch for opposing aircraft.

The F15 can carry as many as 22 bombs of 500 pounds each or five of 2,000 pounds, according to aircraft specialists. But this would leave so little room for fuel that Israel's planes probably flew to Tunisia with much lighter bomb loads to increase their range.