French President Francois Mitterrand held his first meeting with Libyan leader Muammar Qaddafi today amid mounting controversy here over whether Libya has fulfilled its part of an agreement with France to withdraw its troops from the central African state of Chad.

The previously unannounced meeting between the French and Libyan leaders was hosted by Greek Prime Minister Andreas Papandreou and took place on the Mediterranean island of Crete. It drew criticism both from members of the right-wing opposition in France and senior U.S. officials unhappy about any move that would lend greater international respectability to Qaddafi.

After more than four hours of talks, Papandreou told reporters that Mitterrand and Qaddafi had both pledged that their troops would definitely leave Chad. "They basically agreed that not a single French soldier, not a single Libyan soldier will remain in Chad," he said.

Papandreou added that France had recognized Libya's right to "fight in the region" if "any third country" intervened in Chad. Qaddafi told reporters that he "confirmed" Papandreou's account of the talks, Washington Post special correspondent Andriana Ierodiaconou reported from Elounda, Crete.

Papandreou's remarks appeared to imply that a certain number of Libyan troops are still in northern Chad -- despite official French statements to the contrary and a joint French-Libyan communique issued last Saturday announcing that a mutual pullout of French and Libyan forces had been completed. About 5,000 French troops were sent to Chad in August 1983 to prevent the overthrow of the pro-western government in Ndjamena by Libyan-backed rebels.

Claims by Chadian authorities and the U.S. State Department that a considerable number of Libyan troops remain in northern Chad initially were denied by French officials. Today, however, a senior member of Mitterrand's Socialist Party, Claude Estier, conceded for the first time that it was possible that "several hundred" Libyan troops still were in Chad.

"Perhaps there are several hundred Libyan soldiers who have not left. You can count on the president to take this matter up" with Qaddafi, Estier told a noisy session of the National Assembly, at which Mitterrand's decision to meet with the Libyan leader was attacked strongly by the right.

Estier's admission reflected the embarrassment felt by France's Socialist government following the disclosures in Washington and Ndjamena at a time when Mitterrand was secretly preparing to meet with Qaddafi. While acknowledging that there is every reason to be skeptical of commitments made by the mercurial Libyan leader, senior French officials also have made it clear that they differ with the Reagan administration over the best way of dealing with him.

The subject of Chad also came up today at a previously arranged meeting at the French Foreign Ministry between French officials and U.S. ambassador to Paris Evan Galbraith and Assistant Secretary of State Richard W. Murphy.

The American participants in the meeting are understood to have expressed Washington's dissatisfaction with Mitterrand's meeting with Qaddafi -- a leader viewed through Washington's eyes as a major instigator of international terrorism.

Before returning to Tripoli from Crete, Qaddafi told reporters that his meeting with Mitterrand constituted a "foundation stone" for a new chapter in Franco-Libyan relations.

"A feeling of mutual confidence was created here. We're opening a new page," he said. Qaddafi went on to praise both Mitterrand and Papandreou, saying, "I am very happy that there is someone like Papandreou and Mitterrand in the Mediterranean region, because they guarantee peace."

Qaddafi said that Mitterrand had invited him to Paris and that he had invited the French leader to Tripoli. Mitterrand returned home without making any public statement on the talks, which took place at a secluded luxury hotel at the beach resort of Elounda.

The authoritative French newspaper Le Monde quoted French intelligence officials today as admitting that there were still approximately 1,000 Libyan troops -- about 20 percent of the original Libyan presence -- in northern Chad. The officials were quoted as saying that the force did not appear to be in a position to conduct any significant offensive but could help to prevent government troops from recapturing the northern oasis towns of Fada and Faya Largeau from the rebels.

Military analysts here believe that a full pullout of Libyan and French troops would have left Chadian President Hissene Habre in a strengthened position, since his army is generally considered to be better organized and equipped than that of the rebels led by former president Goukouni Oueddei.

Habre summoned French journalists in Ndjamena last night to complain that the Libyans had reentered Faya Largeau after Nov. 5 with tanks and helicopters and reconstituted a defensive line throughout northern Chad. A spokesman for the U.S. State Department said yesterday that "substantial Libyan forces" remained in Chad in violation of the agreement between France and Libya.

U.S. officials said they had provided France with satellite information showing the Libyan forces still in Chad. But spokesmen for the French Foreign Ministry continued to insist today that they stuck by last Saturday's joint Franco-Libyan statement that the terms of September's evacuation agreement had been carried out.

As recently as yesterday, French Assistant Foreign Minister Jean-Michel Baylet assured the United Nations that "Libyan troops have completely withdrawn from Chad."