Libya's expeditionary force was airlifted out of Uganda today, leaving President Idi Amin's crumbling government at the apparent mercy of invading Tanzanian and Ugandan exile troops, according to diplomatic sources.

The evacuation of perhaps as many as 2,000 Libyan soldiers who were Amin's last bulwark marked the likely next-to-last chapter in the slow motion, now five-month-old invasion designed to oust Amin.

Observers predicted the offical fall of the now defenseless and encircled capital of Kampala could be delayed for a day or two.

They reasoned that the Tanzanians, who have borne the brunt of the invasion effort in a war without major battles, would pull back to make it appear to outsiders that the Ugandan exiles had captured their own capital.

The Libyans, most of whom were flown into Uganda serveral weeks ago when Amin's army began its disintegration, left for home from at least two provincial airfields, the sources said.

American-built Boeing 727 jetliners and C130 Hercules military transports were used in the exodus from Nakasongola, an Israeli-built concrete runway air base 90 miles north of Kampala, and Natiswera, a dirt strip a few miles north.

It was difficult to obtain reliable information on the situation inside Uganda since no correspondents have been able to enter the country.

Amin's whereabout were not known. In recent weeks he has kept moving around the country in an effort to rally his disheartened troops and avoid possible assassination. He was last seen Friday afternoon when he arrived by helicopter at Radio Uganda in downtown Kampala to record a defiant broadcast promising to fight to the end.

Telltale signs of the long rumored Libyan pullout emerged late last night when the Libyan ambassador and staff paid their respects to the diplomatic corps and left the capital.

Further signs of disintegration were provided this afternoon by the sudden interruption of telephone service between Uganda and Kenya-restored hours later-and the desertion of yet another high Ugandan official.

The latest to join a good half dozen Ugandan ministers in the Kenyan capital of Nairobi was Chief Justice Mohamed Saieed, a British passport-holder born in what is now Pakistan.

Europeans leaving Kampala this morning said no Libyan-and few Ugandan-soldiers were visible and that the number of raodblocks on the way to the border had been cut from 10 to four.

Following classic military tactics, the invaders attacking Kampala purposely left a single escape corridor open to the Libyan troops and Amin army stragglers.

How the fleeing troops made their way first east, then north to the provincial airfields was not clear. But reports said at least some Libyan troops were seen yesterday taking trains from Jinja, 40 miles east of Kampala.

Diplomatic sources reported that the Libyans were seen carrying an undetermined number of coffins with them. The sources said four Libyan soldiers guarding a Kampala power substation were killed in a shootout recently with Ugandan troops.

Ugandan exile and other sources have reported that the Libyans, a young, scruffy, undisciplined outfit by many accounts, have made themselves unpopular by their behavior.

Just two weeks ago Libya's leader Muammar Qaddafi delivered an ultimatum to Tanzanian President Julius Nyerere warning him to withdraw his invasion force from Uganda or bear the consequences.

If nothing else, the arrival of Libyan reinforcements was credited with slowing down the already painfully sluggish Tanzanian-led advance.

But if Qaddafi did fly in more than a token force, he did not make the major, long term commitment required to drive the invaders out and keep Amin in power.

Diplomatic sources reported that the main Tanzanian units were still about 12 miles from Kampala and that Entebbe airport was abandoned without a fight on Thursday.

One of the rate close-in military engagements of what has been largely an artillery barrage conflict took place Monday when the invaders ambushed a Libyan unit and inflicted heavy losses, according to diplomatic sources.

Why the invaders did not push farther and faster remains a mystery. Some observers are convinced Nyerere kept counting on everything from a spontaneous uprising among Ugandans to Amin's taking fright and fleeing for safety abroad.

For most of the past two weeks Amin has provided the world with a bravado performance as he defied his enemies and critics. He has simply refused to admit he is at the end of his tether.

Also adding to the delay, observers believe, is the lack of unanimity in Ugandan exile ranks that has played so crucial a role in allowing Amin to remain in power for the past eight years.

As the Amin drama plays out what looks like its last act, analysts wondered if the recently formed 11 man executive committee of the Uganda National Liberation Front could over come past differences among its leading members.

Analysts suggested that their best bet for success may well be the knowledge that Byerere, the man who made the invasion possible, is still likely to favor former Ugandan president Milton Onote who was pushed aside recently when the executive committee was formed.

Obote, a nominal leftist who shares many of Nyerere's socialist ideals, has been living in the Anzanian capital of Dar es Salaam since Amin over threw him in a 1972 coup.

"Obote is Nyerere's man," a knowledgeable source said," he hasn't been kept there all these years for nothing."

As if to pre-empt Oboto and establish its won bona fides, the front announced in Dar es Salaam today the establishment of a fund for relief and reconstruction for Uganda. The front has already announced that it was undertaking reconstruction work in "liberated" areas. CAPTION: Map, no caption, By Dave Cook-The Washington Post