France and Libya today announced the completion of a phased withdrawal of their troops from the landlocked African nation of Chad after a 15-month standoff in the desert.

The withdrawal paves the way for a modest improvement between Paris and Tripoli in relations that have been strained severely as a result of the confrontation in Chad.

French officials have hinted at the possibility of a summit meeting between Libyan leader Muammar Qaddafi and French President Francois Mitterrand.

The completion of the pullout represents a significant foreign policy success for Mitterrand at a time when his standing in the opinion polls has sunk to an all-time low because of public discontent at the way his Socialist government has handled the economy.

About 3,000 French troops were sent to Chad in August 1983 to prevent the overthrow of the government by Libyan-backed rebels.

"The purpose of the operation was to make the foreigner leave. He has left. Chad is once again in the hands of the Chadians," said Foreign Minister Claude Cheysson in a television interview.

Questioned about the possibility of improving relations with Libya, Cheysson added: "Col. Qaddafi has a special kind of policy, different from ours. It is good to be able to talk to him, at all levels. Such meetings must take place, also at the highest level, when the occasion presents itself."

The withdrawal of French and Libyan troops was confirmed by observers from both sides. Previously Chadian officials had expressed doubts that the Libyans would stick to their side of the agreement, which was announced Sept. 17.

The pullout leaves Chadian President Hissene Habre in a strengthened political and military position. His French-trained Army is considered better organized and equipped than the rebels under former president Goukouni Oueddei, who still controls the north of the country.

French military intervention in Chad was condemned by radical black African states. But it received enthusiastic support from the Reagan administration and moderate Francophone African countries concerned about the threat of Libyan expansionism to their own stability.

Today's joint statement, released simultaneously in Paris and Tripoli, read: "The evacuation operations of the French forces in Chad and the Libyan units . . . have ended today . . . following the intervention of mixed teams of observers as foreseen under the agreement signed between the two countries."

While acknowledging that Libya enjoys better lines of communication with Chad, which is on its southern border, French officials have insisted that French troops will be sent back immediately in the event of a Libyan return.

They also have warned privately of the possibility of using French air power to thwart any new Libyan move.

Recent comments by French officials suggest that the French government is attempting to use diplomatic pressures to discourage Habre from launching an expedition to reconquer rebel-held territories in northern Chad. The French fear such an attempt could provoke Libyan retaliation and wreck the withdrawal agreement as well as the chance of a negotiated settlement among Chadian factions.

Speaking to the National Assembly last week, Cheysson said that after a civil war that has already lasted 17 years the "legitimate government" in Chad should concentrate its attention on economic development and restoring peace. He also announced $35 million in French aid for Chad, one of the poorest, least developed countries in the world.

Insisting that France did not make a habit of "interfering in the internal affairs of other states," Cheysson said the intervention in Chad differed from both the Soviet invasion of Afghanistan in 1979 and the American invasion of Grenada last year.

" Our military operation is not the same as the American operation in Grenada or the Soviet in Afghanistan. Its objective was to force the departure of the foreigner. He has departed," the foreign minister said.

Political analysts here regard the Libyan pullback from Chad -- the second such withdrawal in four years -- as part of an attempt by Col. Qaddafi to improve his image abroad. In August, the Libyan leader signed a treaty of unity between his revolutionary republic and Morocco, a conservative and generally prowestern kingdom.