An Afghan exile leader who apparently had been living under Soviet protection in Eastern Europe overthrew the Marxist government of President Hafizullah Amin, acording to radio reports monitored today here and in Tehran.

The lightning coup, led by Babrek Karmel, was Afghanistan's fourth in six years. It followed a massive airlift of Soviet troops into Afghanistan earlier this week, ostensibly to help the Amin government battle rebellious Moslem tribesmen in control of much of the countryside.

Radio monitors in Washington quoted Radio Kabul as saying that Amin was executed after a revolutionary court sentenced him to death for alleged crimes against Afghan people.

[U.S. officials said they had received word of fighting around the radio station and the Durulaman Palace in Kabul, the capital, and that Soviet troops appeared to be taking part.]

[In Moscow, the Soviet news agency Tass quoted Karmel as saying that the Afghan people had destroyed "the bloody dynasty" of Amin.]

[The Karmel communique carried by Tass called Amin and his followers the "murderers of tens of thousands of our compatriots, fathers, mothers, brothers and sisters."]

Washington Post correspondent Kevin Klose reported from Moscow that it could not be said with certainty that the Soviets had had a direct hand in the coup, but that Karmel's remarks as reported by Tass seemed to indicate direct Soviet propaganda lines.

Klose noted that Karmel pledged "respect for the sacred Islamic religion," a clear move to placate the Moslem tribesmen whose opposition to Marxist rule was at the heart of the civil war that has plagued Afghanistan since the communist coup of April 1978.

Speculation in diplomatic circles here centered on the possibility that the Soviets had brought in troops in recent days under the guise of an offensive against the Moslem rebels and then helped topple Amin.

During the past six months, the Soviets have poured huge amounts of military hardware into Afghanistan, including more than 100 m24 helicopter gunships that diplomats in Kabul have reported as the most effective weapons against rebels. It was believed until recently that most of the fighting was done by Afghans, with some Soviets serving as gunship pilots or gunners.

Today's events raised the possibility that the Soviets may have used their troops to help resolve an internal power struggle between two feuding factions of Afghanistan's communist People's Democratic Party, which split into two fractions 12 years ago. The two factions have battled each other for power since then.

Radio Kabul reported tonight that Karmel, a former deputy prime minister in the revolutionary government of the late Nur Mohammed Taraki, had taken over the government.

Karmel, who is believed to be about 50, is a founding member of the Marxist Party. He has been exiled in Prague, where he served as ambassador, ever since an intraparty battle about 18 onths ago that left Amin's Khalq (People's) branch in power.

Many observers, however, believe that the Soviets were keeping Karmel and other Parcham exiles in reserve, situated in East European capitals in case it became necessary to replace the leaders of the already pro-Soviet Afghan government. It was not clear exactly when Karmel returned to Kabul. a

Taraki and Amin seized power in April 1978 from Mohammed Daoud, but their pro-Soviet government came under increasing attacks from Moslem tribesmen who opposed it both for its "godless communism" and for its reform measures which the tribesmen thought interfered with their traditional practice of Islam.

Amin took over from Taraki last September after a shootout in the formal royal palace at a time when the Soviets were believed to have forged an agreement with Taraki to ease up on reform measures in an effort to make the regime more popular.

There were reports that Taraki was fatally wounded in the gunfight, but Amin later announced that he had died of natural causes.

At the time, there also were reports that Amin's action had caught the Soviets by surprise. Later, Amin accused the Soviet ambassador in Kabul of helping Taraki set up an ambush for him.

Amin's hard-line policies subsequently seemed to spell only trouble for Moscow, which was seeking to widen the power base of the Kabul Marxists and is now mired in the civil war.

According to observers here, it appears the Soviets are now evening the score with Amin for overthrowing their man, Taraki, last September.

Other observers suggested that the Soviets took advantage of the U.S. preoccupation with Iran to move their troops into Kabul in a giant airlift involving as many as 150 planes.

State Department spokesman Hodding Carter yesterday said, "It appears that the Soviets are crossing a new threshold in their military deployment into Afghanistan."

Some speculated here that the State Department expected the Soviet move because of a line in yesterday's statement which refused to go into Moscow's motives for the troop buildup, but said, "the Soviets are going to have to speak to their motives, but the effect is to increase their interference in the internal affairs of Afghanistan."

According to Iran's official Pars news agency, Karmel promised "democratic freedoms for all the masses, the release of all political prisoners and the creation of jobs for the unemployed" in a radio broadcast.

He also said that he was willing to negotiate with enemies of the Amin government, indicating a possible move for a peaceful end to the rebellion which has left the government holding little more than major cities and roadways while the rebels roam over the countryside.

Afghan governments historically have found it necessary to be friendly with Moscow, but Taraki and Amin moved the country much closer to the Soviet orbit when they took over 19 months ago.

Great masses of Afghanis were alienated, however, by the combination of authoritarian rule that saw thousands of political leaders, reporters, university professors and intellectuals jailed and possibly killed, and others -- such as Karmel -- driven into exile.

For the Moslem tribesmen scattered through the rugged hills and valleys of the Texas-sized country, the attempts to reform traditional marriage and land laws were enough to drive them to open rebellion. Until the appearance of this winter's winds and snows, it appeared that the rebels could at least hold their own using hit-and-run tactics against the government forces, which were backed by Soviet advisers and sophisticated equipment.