Zaire's President Mobutu Sese Seko arrived unexpectedly here yesterday dressed in an elegantly pressed leopard-spot camouflage combat uniform and gold-decorated general's cap. But even his discription of the military situation in his invasion-torn Shaba province failed to galvanize the 21 African countries attending a summit conference here to accept a proposal for a joint African force against aggression.

Gabon's President Omar Bongo, current head of the Organization of African Unity, who presented the proposal, dropped it meeting with a cool reception from several participants, including Mali, Niger and Guinea-Bissau, conference sources said.

The Ivory Coast's Felix Houphouet-Boigny, dean of the 14 African chiefs of state and government attending the annual conference with France, suggested that the way out is for the Africans to continue to depend on France.

"We count on the support of France," he told a press conference at the end of the session. "We have complexes about that. The European members of NATO call upon the United States in case of attack and the states of Eastern Europe call upon Russia. There is no reason that France, faithful to its traditions, should not honor its commitments toward us, that is to say, to come to our aid if we should be attacked."

Apparently as a substitute for Bongo's original idea, it was agreed that the six countries of the West African Economic Community would work toward transforming their present no-naggression pact into a defense alliance open to other African countries.

The Community consists of Ivory Coast, Senegal, Mali, Mauritania, Niger and Upper Volta. Togo is an associate. Bongo told the press conference that the new defense pact might also include his own country, Gabon, plus the Central African Empire, Chad, Cameroon, Zaire, Rwanda and Burundi.

All of the countries are former French colonies except Zaire, Rwanda and Burundi, which were once Belgian colonies.

The list is essentially that of the 14 states represented here by heads of state or government. Seven other countries will lower-level representatives here tend to be offshore island republics or countries like Marxist Guinea-Bissau that do not fully identify with France's main African allies.

The 14 drafted a resolution thanking France for its intervention in Zaire and calling on the conference host, French President Valery Giscard d'Estaing, to express their worries about aggression from outside Africa when he goes to the United Nations and to Washington this week, conference sources said. But opposition from some of the other seven blocked adoption of the resolution.

The 14 African leaders held a separate session yesterday afternoon to hear Mobutu.

When reporters at the airport tried to get Mobutu in his spiff' uniform, to say how long he wanted French forces to remain in Zaire, he sidestepped the question. "You see under what conditions I am arriving. I was at the front."

He said he had come at the unanimous insistence of his African colleaguesto describe the military situation.

Little word of Mobutu's presentation, which he gave in a business suit, filtered out of the meeting confined to the 14 African leaders and Giscard, but the Zairian head of state called a press conference for today.

Giscard, who presided at yesterday's press conference flanked by Houphouet and Bongo, said that the French Foreign Legion unit operating in the Kolwezi region of Zaire would be brought home after it has finished its mission of searching for missing persons and trying to rescue any hostages.

He said that Zaire would be informed in plenty of time so that it can provide its own security in the region. Mobutu has asked France to keep its forces there for an unspecified time.

Giscard said that when he sees President Carter this week he will urge the United States and the rest of the West to sharply increase development aid to Africa to counteract the kinds of dissatisfaction that provide fertile ground for aggression.

Giscard announced increases in French aid for the drought-stricken countries surrounding the Sahara, help for dam projects on the Senegal River and the creation of an African Solidarity Fund with headquarters in Niger.

Houphouet said that Africa was fighting on two fronts at once - the economic and the military.

He said Africans should agree among themselves that the only superpower competition to be tolerated on the continent be economic, social, cultural and technological.