Israel's supporters on the Senate Foreign Relations Committee yesterday assailed the effectiveness of international safeguards against the spread of nuclear weapons as they defended last week's Israeli air attack against Iraq's research nuclear reactor.

During a day-long hearing on the raid, Sen. Alan Cranston (D-Calif.), one of the most outspoken of Israel's defenders, said he had received from American sources four International Atomic Energy Agency documents indicating that Iraq could have produced up to three nuclear bombs a year at the reactor without detection by the IAEA.

Cranston also said the Roger Richter, an American inspector who resigned from the IAEA earlier this week, will testify before the committee today that IAEA safeguards are "totally incapable" of detecting the producation of weapons-grade plutonium at reactors such as the French-built Iraqi facility.

Cranston's office later released three of the documents, none of which specifically mentioned the Iraqi reactor. Two were IAEA memoranda discussing the difficulty of devising a system to detect the secret production of plutonium at nuclear reactors.

The other document, unsigned and undated, purported to be an analysis of the plutonium-producing capabilities of a reactor such as the facility near Baghdad.

Cranston's assertions contradicted the opinion of Dr. Sigvard Eklund, the IAEA director-general, who said last week that it was "practically impossible" for the Iraqis to be mading plutonium for weapons at the reactor.

Crantson's criticism of the safeguard procedures also appeared to be at odds with the opinion of State Department officials. Under questioning about the Iraqi facility, John P. Boright, acting assistant secretary of state for oceans and international environmental and scientific affairs, said: "Generally, our view is that that kind of reactor can be safeguarded."

But Undersecretary of State Walter J. Stoessel Jr. and other administration officials who appeared before the committee also conceded that the United States has long been concerned about the long-term possibility that the reactor would give Iraq the capability of building nuclear weapons.

The issue of nuclear proliferation dominated the day of hearings, during which Israeli Prime Minister Menachem Begin was criticized by two senators.

"We must explore," said Sen. Charles McC. Mathias Jr. (R-Md.), "whether the best interests of Israel and the United States are served by standing idly by while the Israeli prime minister pursues actions which move Israel furthr and further from rapprochement with its Arab neighbors."

Sen. Paul E. Tsongas (D-Mas.) criticized Begin for his "knowing affront" to Egyptian President Anwar Sadat, with whom he met shortly before the raid without giving any indication that Israel was about to embark on a military misssion.

Sen. John Glenn (D-Ohio) labeled the Israeli air attack a "vigilante tactic," and said he was asking President Reagan to convene a meeting of nuclear supplier nations in order to put teeth into non-proliferation Policies.

Israel contends that its raid was defensive because Iraq planned to use the reactor to produce nuclear bombs.

Stoessel yesterday repeated the administration's position that Israel had not exhausted diplomatic options in dealing with the Iraqi nuclear facility and that the air attack "could not but seriously add to the already tense situation in the area and seriously complicate our effort to resolve the various problems in the area through peaceful means."

But Stoessel said this criticism did not mean the administration has concluded that the use of American-supplied weapons in the raid constituted a violation of Israel's arms agreements with the United States. The adminstration is reviewing that question and, in the meantime, has suspended the delivery of four F16 fighter-bombers to Israel.