The Middle East crisis demonstrates the weakness of Soviet-American detente and highlights the need for firm American diplomatic policy, an exiled Soviet dissident told a Science Center audience last night.

Dmitri K. Simes, a political scientist who carried out domestic protests against Soviet emigration policies for Jews before leaving the Soviet Union in January 1973, spoke in a lecture jointly sponsored by Harvard-Radcliffe Hillel, Social Democrats USA and the Youth Committee for Peace and Democracy in the Middle East.

Simes, presently at Georgetown University, said the Soviet Union decided that detente with the United States could be sacrificed for support of the Arabs. This decision resulted from Soviet fears that the Chinese would replace them in the Third World if they did not back the Arabs, Simes said.

Weak American response to the Soviet's military build-up of the Arab nations encouraged the Soviets in their course, Simes added.

"Some people blame Chairman Brezhnev," Simes said. "I will blame Mr. Kissinger." The muting of American protest was interpreted by the Soviets as weakness, Simes added.

When the war erupted, the United States was right to adopt a cautious approach, Simes said.

Surprised and Disturbed

"At the same time, I was surprised and disturbed that the United States was ready to put pressure on Israel and the friends of the United States and not on their enemies," he added.

Simes attacked Sen. William J. Fulbright (D-Ark.) for advocating U.S. interference in Israel's domestic affairs and contrasted that stand with Fulbright's opposition to a "detente with civil rights" that would mean "interference in Soviet domestic affairs."

The military alert which President Nixon called was necessary, Simes said, adding that there "was enough evidence that the Soviet Union considered the situation serious."