U.S. Ambassador Donald F. McHenry told the U.N. General Assembly that the Soviet Union's explanation for invading Afghanistan was an "insult" to the intelligence of the world community.

McHenry echoed statements made yesterday by Secretary of State Cyrus Vance, saying that "the United States and other nations are considering . . . unilateral and multilateral measures to convince the Soviet Union of the magnitude of its error, including withdrawal from cultural exchange, trade missions and even the 1980 summer Olympic Games."

By its adjournment early this evening, more than 50 nations had spoken in the third day of an emergency General Assembly session convened to debate a resolution condemning the invasion and calling for the withdrawal of "all foreign troops" from the Central Asian country.

With the exception of India, which yesterday described the Soviet Union as a "friendly country" seeking only to repel other foreign aggressors from Afghanistan, no noncommunist nation has defended the Soviet move.

A series of Third World and European delegations have condemned the invasion and labeled it a grave threat to world peace.

In his speech this morning, McHenry told the assembly that "it is imperative that we demonstrate we cannot be duped into ignoring our responsibility to defend the principles on the [U.N.] Charter."

McHenry referred to Soviet assertions that its troops' presence was requested by the Afghan government to drive out "armed intervention" from the West as "tortured explanations that insult our intelligence."

The Soviet invasion, he said, is "no cold war squabble" as the Soviets have charged. "It is not the United States whose freedom is most threatened by Soviet indifference to the Charter," McHenry said. "The small and the nonaligned countries, like Afghanistan, are most imperiled."

No assembly debate is scheduled for Sunday. The emergency session is expected to resume Monday, with a vote on the resolution condemning the invasion anticipated early in the week.