Leaders of the Federation of American Scientists and five Americans who won Nobel prizes in science have signed pledges refusing to participate in official scientific exchange programs with the Soviet Union until dissident Soviet scientist Andrei Sakharov is freed from exile, the federation announced yesterday.

"It is a very big step for a scientist to make -- to cut himself off from a very sophisticated portion of the world scientific community," federation director Jeremy J. Stone said.

Two observers from the Soviet Embassy attended the press conference. Asked later what effect the pledges might have, one said: "For American science, I think it is very bad."

Asked what the effect might be on Soviet science, the official paused, smiled and said: "Let's see."

The action by the federation -- with 5,000 members, including 37 Nobel laureates -- came two days Rep. George E. Brown Jr. (D-Calif.) introduced a resolution in the House calling on the United States to suspend scientific exchanges with the Soviet Union for a year.

Stone said he opposed such blanket action, believing that noncooperation pledges by individual scientists is more effective.

"We recognize, as we always have, the importance of maintaining the scientific brotherhood," Stone said.

"And we do not mean, in any case, to exclude personal contacts aimed at diplomatic solutions of war and peace issues and other nonscientific questions or the exchange or reprints and so on."

He said that the Soviets could interpret a cutoff of scientific exchange by Congress as "a politically hostile act" and not a true expression of feeling by American scientists.

The federation has protested Soviet treatment of dissidents before but had not gone so far as to suggest that its members consider refusing to participate in exchange programs.

In another development, the presidents of five leading universities formally protected the effort to silence Sakharov's "clear voice of reason."

A telegram sent to Soviet leaders said Sakharov's banishment to the closed city of Gorki "will further exacerbate strong feelings against the Soviet Union. We urge you to reconsider your harsh treatment against one of the world's great human beings."

The protest was signed by presidents Derek C. Bok of Harvard, Marvin L. Goldberger of the California Institute of Technology, Richard W. Lyman of Stanford, David S. Saxon of the University of California and Jerome B. Wiesner of the Massachusetts Institute of Technology.