French President Francois Mitterrand acknowledged publicly today for the first time that some Libyan troops have remained in Chad despite a series of official statements confirming the full withdrawal of French and Libyan forces from the landlocked African state last week.

The French leader, back from a previously unannounced meeting yesterday with Libyan leader Muammar Qaddafi on the Greek island of Crete, estimated Libyan troop strength in northern Chad at about 1,000 men -- or a little less than one-fifth of the estimated Libyan presence before the pullout began.

"The Libyan military presence in Chad . . . is less than is being said by certain foreign information sources but greater than there should be," Mitterrand told journalists in an apparent reference to reports in Washington and the Chadian capital of Ndjamena that have put the number of Libyans remaining in Chad much higher.

Secretary of State George P. Shultz has declined an invitation to attend a dinner given by visiting French Foreign Minister Claude Cheysson in Washington Monday, according to a U.S. official in Washington who suggested that irritation over Mitterrand's meeting with Qaddafi was the principal cause of the unusual decision not to attend.

Officially, Shultz has told the French that he had other commitments, the source reported. The State Department's decision to publicize intelligence information about the Libyans remaining in Chad as Mitterrand was preparing to meet Qaddafi has caused strains between Paris and Washington, U.S. and French officials reported.

Mitterrand's remarks in Paris appeared designed to defuse a growing public controversy here over the way his Socialist government has handled the pullout from Chad, originally announced in a joint Franco-Libyan communique last September and officially reported completed Nov. 10. About 3,000 French troops were sent to Chad, which lies on Libya's southern border, in August 1983 to bolster the pro-western government against a force of Libyan-backed rebels advancing from the north.

Commentaries today in many French newspapers, including some that are normally sympathetic to the left, expressed bewilderment at Mitterrand's decision to meet with Qaddafi at this particular moment and fears that the president may have been duped by Tripoli. Official French spokesmen who as late as yesterday were still talking about a full Libyan retreat have been held up to public ridicule.

Comments by Mitterrand and other French officials today strongly implied that the French government knew that some Libyans remained in Chad at the time it issued a joint statement with Tripoli on Nov. 10 confirming the completion of "the evacuation operations." Speaking to journalists at the Elysee Palace today, the president said that the Libyan withdrawal had been proceeding normally up until Nov. 9 or 10, when it was possible to observe "on the ground either a slowing down in the movement of withdrawal or a reinforcement of the Libyan presence."

Government spokesman Roland Dumas told a previously arranged luncheon hosted by the Anglo-American Press Association that the French government had access to its own sources of information about Libyan troop movements even before acquiring U.S. satellite data from Washington earlier this week. He denied that the French authorities had received copies of pictures taken by the satellites but confirmed the receipt of information based on the photographs.

Dumas confirmed that France stood by a statement made in September by Cheysson threatening to send its troops back to Chad in the event of any Libyan return. "If they leave, we leave. If they stay, we stay. If they come back, we will come back," the foreign minister said.

According to French officials, there are about 100 French soldiers still in Chad -- serving as "military instructors" with the government Army. Some heavy French military equipment, including tanks and armored cars, also has been left behind.

French spokesmen refused to confirm a statement by Greek Prime Minister Andreas Papandreou that Mitterrand and Qaddafi reached agreement on the full withdrawal of all French and Libyan troops.

"We can't say there was an agreement. There was an exchange of views about this problem," Dumas said.

Describing the tripartite meeting as "historic," Papandreou said that both sides had accepted the principle that "not a single French soldier or a single Libyan soldier" should remain on Chadian territory. He added that France had "officially recognized" Libya's security interests in the region by recognizing Tripoli's right to intervene in the event of a similar intervention by "a third country" in Chad.

In his unusual press briefing today, Mitterrand said that the Libyans still in Chad did not have any heavy arms or significant air support apart from several helicopters. He said that the movement toward withdrawal was continuing.

The president said that Libya could hope to restore normal relations with France only when all its troops were withdrawn from Chad.