A senior Soviet adviser has flatly rejected any international call to replace the Marxist Afghan government, although he concedes that it is highly unpopular. Replacement of the Soviet-installed Kabul leadership is a key part of recent American suggestions for reaching a political settlement of the Afghan crisis.

"This is a nonstarter which leaves no space for negotiations," said Georgi Arbatov, a nonvoting member of the Communist Party Central Committee and a senior authority on the Kremlin's relations with the United States."If you want to change the government, then don't speak about a peaceful settlement, please," he asserted in an interview with The Washington Post.

Arbatov also said last Sunday's announced withdrawal of a division of Soviet troops and some tanks had brought an overall drop in Soviet military manpower in the country, estimated in the West at about 85,000. "The level of troops has declined. This isn't some game [played] after others sneaked in. It is an honest withdrawal," he said, and should be seen as a Kremlin gesture of interest in a political settlement.

However, he left the door open for a possible future troop increase. "I hope [the withdrawal] is finite. It depends on how the situation goes."

Ever since the Soviet invasion on Dec. 27, senior Soviet officials have had minimal contact with Western correspondents, Arbatov agreed to be interviewed after the Central Committee general session Monday, which took the unusual step of reviewing the Soviet Union's international situation and approving a resolution restating Soviet support of detente.

But everything Arbatov had to say in his 80-minute conversation made clear that the Soviets have no intention of abandoning the military intervention that secures the Kabul government of Babrak Karmal. He sought to minimize the Soviet action as a "local problem" that must not be allowed to disrupt the superpower dialogue over what he said are far more important matters, such as strategic arms control.

Arbatov's language lacked the harshness of recent official media attacks on President Carter and his administration, but he was just as unyielding in blaming the United States for the world tensions enveloping major capitals. He insisted that the Kremlin could not be blamed for any of this and that its actions have no aggressive intentions.

He spoke as head of the Institute of the U.S.A. and Canada, Moscow's main think tank on American affairs. He said the strong anti-Soviet mood in the United States has been abetted by the presidential race. It has "become much safer to run against the Russians in this election year. All in all, this is a serious situation which makes the United States a difficult partner. Ronald Reagan says bad things about us and President Carter does them." He asserted that greater "political courage" could stem the tide of U.S. anxiety about the Soviet Union.

With greater candor than has been heard here to date, Arbatov conceded that the Marxist leadership in Afghanistan is bitterly unpopular. "It is almost inevitable that revolutions make many mistakes and here maybe they made much more than the usual amount of mistakes, which led to very serious disagreements."

But he maintained Moscow's line in its test of wills with Washington over the invasion that the Moslem revolt in the Afghan countryside is supported directly by the United States, China, Egypt and Saudi Arabia. "We know how it can be simulated as local resistance when the main bulk of it comes from some small but well-organized groups abroad who play on religious fanaticism," he said.

The Soviet demand that alleged external aggression and subversion cease before any full military withdrawal are "two very legitimate conditions," he said. "We don't plan to have permanent troops there. We don't regard Afghanistan as some sort of springboard for military incursions into other areas."

Arbatov said the fact that the United States maintains an embassy in Kabul means Washington recognizes Babrak as the legitimate leader of the country, as it did his Marxist predecessors, Nur Mohammed Taraki and Hafizullah Amin. Both men died at the hands of their successors. The United States has not named an ambassador to Afghanistan since Adolph Dubs was kidnaped by Moslem rebels and died during an attack on his captors by government security forces in February 1979.

To call the strife a spontaneous uprising against the Marxists, he said, is a "Western version of events. We don't agree with them. There are just two different pictures."

He asserted again the Soviet position that serious bilateral deterioration set in months before the intervention. "SALT II [the pending strategic arms limitation agreement] was almost dead," he said, America had improved ties with China and was discussing arms deals with Peking, had excluded Moscow from a Middle East peace effort despite a mutual pledge to work together, and Carter had decided to increase U.S. arms spending.

"We already had arrived at a very bad juncture before this -- we read the situation this way -- and in my opinion this played a certain role in the Afghan decision. It had two meanings: to sustain the legitimate government and [respond to] our security concerns."

He said the rebellion threatened to "open more than 2,000 miles of our border and borders with China," the Soviets' archrival. "With the U.S. Navy massed in the Indian Ocean, we somehow couldn't believe this was all just to save the hostages."

Arbatov rejected the argument that U.S. arms spending increases and the NATO decision to upgrade its nuclear missile forces were a response to continued Soviet military buildup. The important controlling point, he claimed, is the strategic parity agreed to under SALT II.

"It was agreed we shall have no superiority over each other. So no one has to complain about it. I don't think a single American general would like to exchange the geopolitical situations, the strategic arms, rockets and missiles of the United States for [those of] the Soviet Union."

As for Republican presidential contender Reagan's recent suggestion that SALT II be scrapped and the United States open a direct arms race with the Soviets, Arbatov declared, "Maybe we have a smaller economy but we can suffer more if we see that we need to do it.

"I would say there is no chance at all to acquire meaningful superiority for the U.S. or the Soviet Union. And I also think these [Reagan] experts are not very good experts on the American economy. Because the American economy is entering not into a fat period, and an open-ended arms race really would be just as difficult for the U.S. as for us."