The victory of Libyan-backed forces in the long battle for Chad's capital city gives Col. Muammar Qaddafi his first significant military victory and represents a major setback for France, the main representative of Western interests in the central African region.

The takeover in Ndjamena by Qaddafi's allies appeared to put the Soviet-backed Libyan leader strategically between the Sahara Desert and black Africa in good position to project his influence across Chad's long, open borders with Niger, Nigeria, Comeroon, Central Africa and Sudan.

France, which had solemnly warned on Saturday against continuation of the Libyan intervention and pronounced French readiness to support "any collective effort that the African states may undertake," stood by apparently hopeless after the disintegration of the capital's defenders yesterday under a week-long onslaught by Libya's tanks, planes and infantry.

Chadian Defense Minister Hissene Habre, who led the forces resisting the Libyan advance, fled early yesterday across the Shari River to Cameroon, where he signed "with reservations" a cease-fire in the civil war.

[Agence France-Presse reported that Habre vowed to continue his struggle against the government because. he said in Yaounde, the Cameroon capital, it is "illegal" and "illegitimate."]

Earlier, when he seemed to have the upper hand, Habre had refused to sign a cease-fire worked out under the auspices of the Organization of Africa Unity. It nevertheless was signed Nov. 28 by his main rival, the Libyan-backed Chadian president, Goukouni Oueddei.

The accord provides for neutralization of the capital, withdrawal of all military forces to a distance of 62.5 miles from the city and its control by an inter-African force. Now that Habre has also signed, refusal by the Libyan-controlled Islamic Legion to withdraw could provide a new legal basis for African-sanctioned action.

Serge Maffert, a columnist for the Paris newspaper Figaro, who is particularly close to the government, suggested today that France would accept a clear mandate from the OAU to station peacekeeping forces in the Chadia captial.

"We are mere spectators in Chad today," a French official said. "There is no legal basis today for a French intervention in Chad."

Officially, Libya says it has no forces in Chad, only military technicians helping the Libyan-recruited and trained Islamic Legion. French officials estimate that there are 4,000 to 5,000 Libyan troops in the huge underpopulated country more than twice the size of Texas. The French claim about 2,500 of the Libyans are in Ndjamena, a town of 400,000 just south of the frontier between the Saharan desert that covers two-thirds of Chad and the fertile, black-populated south where most of the 45 million Chadians live.

The Libyan victory is recognized here as a major logistical feat of the sort that the Libyan armed forces were not throught capable of. They transported 50 to 60 heavy tanks and heavy artillery pieces south across 600 miles of desert. This success was in sharp contrast with most previous Libya military efforts, such as Libya's disastrous military expedition in Uganda in which hundreds of Libyan troops were killed, wounded or captured.

Habre's well-entrenched but relatively lightly armed forces, generally considered one of the best disciplined and well-trained military organizations in French-speaking Africa, were no match for the Libyan tanks.

French officials say that the withdrawal of Habre's forces was not a rout, but an orderly retreat by road by about 2,000 men with their military trucks northeast to his fief of Abeche, near the Sudan border.

Goukouni's forces claimed to surround that town. But it is the gateway to mountainous country that Habre has proved in the past he knows how to use to maximum advantage for guerrilla warfare.

That prospects leads many Western analysts here to believe that the Libyan triumph may be short-lived. There Chadian allies are prone to sudden reversals of alliances.

The main Chadian ally of Goukouni, who has fought the Libyans in the past when they were backing Habre, is the force of Vice President Abdelkader Kamougue, made up of southern animists and Christians who are the natural enemies of Qaddafi's Islamic and Arab followers. Most analysts here expect rapid dissension in the winning coalation.