An informal group of well-known American scientists revealed yesterday that more than 2,400 colleagues have signed personal pledges to end or restrict their cooperation with Soviet scientists until the Soviet Union releases two famous political dissidents from prison.

This unexpected announcement -- a surprise both to the State Department and the National Academy of Sciences -- represents an escalation of organized protest by U.S. scientists of Soviet harassment of dissident academics.

Several officials of scientific organizations said last night that the new protest could have a measurable effect on the level of Soviet-American scientific cooperation, an area in which Soviet authorities have shown unusual sensitivity in the past.

The two dissidents to whom the 2,400 Americans have lent their support are Anatoly Scharansky and Yuri Orlov. Scharansky, 31, was sentenced in July to 13 years at hard labor for "high treason in the form of espionate." Orlov, the highest ranking scientist sentenced to prison in the Soviet Union since Stalin's time, is a high-energy physicist and former member of the Armenian academy of sciences. He was sentenced last May to seven years at hard labor and five years of "internal exile" on charges of anti-Soviet activity.

Orlov, 55, was chairman of the "Moscow Helsinki Watch Group," an unofficial dissident organization set they began to think that the Soviet might not move on SALT as long as China's invasion continued.

Carter invited Dobrynin to the White House Tuesday afternoon to try to "stabilize" Soviet-American relations by reassuring the ambassador that the United States did not approve of China's invasion, and did not want events in other parts of the world to disrupt the Soviet-American dialogue, according to an informed source.

But before that meeting took place, Dobrynin met Vance Tuesday morning to convey Soviet responses to the latest U.S. SALT proposals. Administration officials reportedly interpret this action as an indication that the Soviets share the same desire to continue doing business with the United States despite the strains caused by events elsewhere.

Dobrynin's new positions do not resolve all the outstanding issues in the SALT talks, and the Special Coordinating Committee of the National Security Council will have to meet next week to decide how the United States should now respond, sources said.

In ordinary times the United States might move more quickly, but with Israels prime minister in Washington, a war in Indochina, tumult in Iran and more, "our circuits are overloaded," as one senior official put it.

Apparently the key remaining unresolved issue involves Soviet encoding of singals sent back to earth by its missiles during test flights. In July 1978 and again in December, according to U.S. sources, the Soviets did "encrypt" such signals in an apparent effort to prevent the United States from learning details of test flights.

The United States has insisted that the Soviets accept that such encryption is inconsistent with the SALT II treaty being negotiated. Thus far the Soviets have accepted a general statement that encryption can interfere with each country's "national means of detection," the phrase used to describe Soviet and American monitoring of each other's activities. The SALT agreement forbids any interference with national means of detection.

But the Soviets have not specifically accepted the U.S. view that their July and December tests violated the standards of the SALT pact. The Soviets suggest that any dispute about specific cases be referred to the Soviet-American Standing Consultative Committee, which oversees SALT pacts.

The United States must decide how far it wants to press this issue. Many senators have said they regard encryption as an important, sensitive issue.

Dobrynin offered a new Soviet position Tuesday on the definition of "new types" of missiles under the SALT agreement. The SALT II pact would permit only one "new type" of land-based missile on each side. The disagreement is over the definition of a "new type," which the United States said should be any missile whose basic characteristies -- left weight, thrust, acceleration and so forth -- are more than 5 percent greater or lower than those of an existing missile.

At a negotiating session in Geneva in December, the Soviets unexpectedly proposed that the standard be 5 percent greater or 20 percent lower than existing missiles. This led some U.S. planners to suspect that the Soviets might use the expanded leeway to deploy a series of smaller missiles that would not be defined as "new types."

On Tuesday Dobrynin proposed a standard of 5 percent greater to 10 percent lower than existing missiles as the new types definition. U.S. sources indicated a belief that this brings the two sides so close together that an agreement is well within reach.

Other technical differences have also been al but finally resolved, sources said.

The location of a summit meeting could still be a stumbling block. Until recently the United States assumed, and the Soviets did not demur, that a summit would take place in Washington, since the last two Soviet-American summits took place in Moscow and Vladivostok.

But the Soviets have recently invited Carter to come to Moscow instead. If that is not possible, they have indicated an interest in Geneva. U.S. officials reportedly still favor a U.S. summit, and have suggested Hawaii and Alaska as well as Washington.

U.S. analysts are not sure if the Soviet attitude is motivated by concern for Brezhnev's frail health or a desire not to come to Washington to bless a SALT pact -- and commit Brezhnev's personal prestige to it -- when Senate approval of the pact remains in doubt.