Belgian and French Foreign Legion paratroops - supported by the United States - were being airlifted to Zaire yesterday in an international rescue mission to evacuate 2,500 Europeans caught in the midst of fighting between rebels and government troops in Zaire's Shaba copper belt.

Waves of Hercules C130 transports loaded with an estimated 1,750 Belgian soldiers took off from Brussels' military airport during the afternoon, reportedly bound for the Ivory Coast on their way to Kamina, a former Belgain base 120 miles north of Kolwezi, where most of the foreigners are stranded.

At the same time, five planeloads of French Foreign Legionnaries reportedly left from Solensara, Corsica, aboard a fleet of transports, also reportedly bound for Abidjan, Ivory Coast, en route to Zaire.

The elite Legionnaire paratroop unit took off aboard DC8 jetliners chartered from private air lines, according to state radio broadcasts.

Neither government gave any indication when the rescue operation would actually begin inside Zaire, although it was expected sometime overnight.

The governments were also secretive about the tactical aspects of the mission, including where the paratroopers would utimately be landed. Zairian troops have recaptured the vital airport at Kolwezi, and presumably the rescue force could land there.

President Mobuto Sese Seko took two dozen Western reporters to the airstrip yesterday to prove it was in government hands. Associated Press correspondent Serge Schemann reported that smoke was reported visible over Kolwezi, five miles away, and the thud of mortars and the rattle of automatic weapons fire could be heard nearby.

The Belgian rescue mission - the second that Belgium has launched in its former colony in 14 years - was ordered by Prime Minister Leo Tindemans following a night-long Cabinet meeting and a later "crisis commitee" meeting that Tindemans said involved France, Great Britain, the United States and Belgium.

In announcing the operation, Tindemans cited reports from the war zone that rebel invaders in Shaba Province were engaged in a 'hunt against whites" and had killed a number of Europeans.

The extent of the U.S. involvement in the operation - such as whether U.S. pilots are being used or U.S. equipment supplied - remained unclear last night, with State Department and Pentagon officials resolutely saying that no details will be disclosed until after the rescue operation actually begins in Zaire.

Pentagon officials last night said that they expected the United States to supply fuel for the Belgian and French planes in Zaire. and reports from Zaire indicated that such assistance was under way. But officials stressed that U.S. military personnel were not expected to remain on the ground or assist the operation directly.

As the operation was under way, the White House announced the granting of $20 million in emergency aid to Zaire in "the national security interests of the United States." Press Secretary Jody Powell said Zaire was "a moderate government" which has "supported our goals" in Africa and is now threatened by an incursion from outside its borders.

"It would be an unfortunate lesson to the rest of the world if we failed to respond to a reasonable request in such a situation," he said.

There were strong indications that some U.S. officials were eager for Washington to participate in rescue operations and to aid Zaire in order to show that the U.S. maintains the capacity and will for quick action to shore up friendly governments under challenge in Africa.

A usually informed source said that Gen. Alexander Haig, the NATO commander, is playing a role in coordinating the rescue operation.

The presidential determination that aid to Zaire is in "the national interest" was required by a rider to the 1977 Foreign Military Assistance Act. This may have been among the measures which prompted President Carter to complain to lawmakers early this week about restrictions on executive action abroad.

White House officials would not say how the "non-lethal" spare parts for C130 transport planes, communications equipment, medical supplies and petroleum, would get to Zaire. They did not rule out the possibility that U.S. aircraft and personnel would fly it in.

As the airlift got underway, about 1,500 troops of the American 82nd Air-borne Division remained in a precautionary state of alert at Fort Bragg, N.C., last night.

They had been ordered readied by Carter for a possible mission to rescue about 90 Americans, mostly contruction employes. A helicopter-and-truck rescue mission by Morrison-Knudson International, the construction firm, subsequently evacuated all but 14 of the Americans.

The Americans who missed the pickup include eight missionairies, a tourist, a copper company employe, and two Morrison-Knudson employes and two dependents.

Besides the Americans and 2,000 Belgians, civilians reported left in the war zone include about 400 French and an estimated 100 persons of other nationalities, including Japanese.

At least 10 Belgians and one Italian have been reported killed in the Kolwezi area since the Angola-based insurgents seized it last Friday after infiltrating through northern Zambia.

In announcing the rescue mission, which officials of every country involved described as essentially a Belgiam operation, Tindemans told his nation that time was running short for the evacuation of the civilians and that "fighting is taking place in the streets of Kolwezi, and whites are the main targets."

When asked by reporters before the operation whether decisions were being made elsewhere besides Belgium, Tindemans replied: "The decision, when it is a question of a military operation, still rests here in our country - it is, of course, purely national, a humanitarian operation, purely national."

He added, "But this is at least a four-way operation . . . Ths entails cooperation with the United States, France, Great Britain and Belgium, and probably some African states. Coordination is taking place now."

The first reaction of representatives of the Zaire rebels was one of bitterness, with a spokesman for the Congolese National Liberation Front saying, "It is disgracful to see the Europeans running to protect their vested interests and to protect the Mobutu dictatorship."

The Soviet news agency Tass said the operations mean that the United States "is stepping up intervention in the internal developments now taking plane in Zaire."

This was the second time since November, 1964, that Belgium launched a rescue mission, with U.S. support to evacuate foreigners from Zaire.

Then, troops aboard U.S. aircraft landed in Stanleyville, now called Kisanmni, in northern Zaire and exacuated 1,400 Europeans held hostages.

The new U.S. military aid to Zaire, Carter announced in a memorandum released yesterday. will come in two segenents, with $2.5 million in the training of high-level Zairian general staff officers in the U.S. to occur over a period of a year. The training is part of a continuing program operated by the United States, but the Zairian partciption had been cut off by the 1977 amendment.

The $17.5 million in "non-lethal" maurial, the White House said will begin moving to Zaire within a week. The shipments of the spare parts, fuel medical supplies and communications equipment will not exhaust the entire $17.5 million, Powell said but the entire allocation must be authorized to permit spending any part of it.

Also contributing to this story were Washington Post staff writers Edward Wash and George C. Wilson in Washington, staff correspondent Renald Kman in Paris and special correspondent John Robinson in Brussels.