Sen. Paul Laxalt (R-Nev.) said yesterday that his diplomatic mission to Manila had persuaded Philippine President Ferdinand E. Marcos that President Reagan is concerned about the military, political and economic stability of that republic.

Denying that Marcos had rebuffed his message, Laxalt said the Philippine leader had "listened carefully and is now absolutely convinced of the president's concerns." Laxalt said Marcos, who has ruled for 20 years including a decade under martial law, had promised "full and fair" elections in 1987 without conceding that any previous elections had been unfair.

The senator, a friend of Reagan and general chairman of the Republican Party, delivered what U.S. officials described as a particularly blunt message expressing growing concern that the Marcos regime will be overwhelmed by a combination of economic and political problems and a growing communist insurgency.

Laxalt said he was chosen to carry the message, which Marcos said afterward provoked a "frank exchange," so it would be unmistakable that the warning came from the president rather than lower-level officials. Another warning was conveyed to Marcos during the week by Adm. Roland Hays, new commander of the U.S. Pacific Fleet.

The two largest U.S. military installations outside the United States are Clark Air Base and Subic Bay Naval Base, leased by the United States from the Philippines through 1991. The Defense Department has proposed to spend an additional $1.3 billion on the installations, a plan that has been criticized in Congress because of U.S. intelligence assessments that the communists might ultimately topple the Philippine government, then demand that the United States give up the bases.

Laxalt said Marcos told him that the communist insurgency was "manageable" and gave him details to take to Reagan about progress in the conflict.

"It is not true that the communist rebels are winning the war," Marcos said Friday night in an interview on ABC News' "Nightline." "They are surrendering in droves."

Estimates of the strength of the communist New People's Army range from 10,000 to 15,000. Marcos disputed the higher figure in the interview.

Laxalt said he will deliver a letter to Reagan from Marcos responding to his concerns as well as a report on the insurgency and economic conditions.

The senator said he had not intended to comment on his mission until after he had reported to the president but broke his silence after calling Reagan because of a story in yesterday's New York Times that said Marcos had rejected a plea for major political, economic and military changes.

"The thrust of the story is inaccurate," Laxalt said.

Neither Laxalt, who termed his mission "a complete success," nor the U.S. officials who briefed him before his trip expected Marcos to announce policy changes because of the visit. One official said it was unlikely that Marcos would have made any public concession because this would have made him appear subservient to the United States. For this reason, Laxalt discussed various issues with Marcos, including purported military corruption and political repression, without requesting specific changes, officials said.

A State Department official said Marcos had indicated neither rejection nor acceptance of the message and that it was unclear what the ultimate impact of the Laxalt mission would be.

But the official said it was evident that a new awareness had been aroused in Marcos about the extent of Reagan's concern for the Philippines.

Marcos expressed this understanding in the Nightline interview, even while defending the performance of his regime.

"Of course they should be concerned because the controversial reports have confused everybody," Marcos said of the views of Reagan and high-ranking U.S. officials.

In the interview Marcos expressed pride in the performance of government troops and their ability to crush the insurgency and proclaimed, "We're not another South Vietnam."

Reportedly, in the document that Laxalt said he will deliver to the president Monday or Tuesday, Marcos claims that he has decentralized the military command to combat the rebels, responding to one of the persistent criticisms of the way he is waging the guerrilla war.

Laxalt also will give Reagan a positive report about the physical and mental condition of the 68-year-old Marcos, who appeared vigorous in the interview with ABC.

"All of the reports about him being a doddering old man were wholly contradicted by my observation of him," said Laxalt, who held four hours of discussions with Marcos.

The Nevada senator, who returned to the Washington area late Thursday, emphasized that he is not a diplomat and was not passing judgments on the validity of Marcos' military, political or economic claims. But Laxalt said it was clear that Marcos took seriously the objections to his policies and prepared rebuttals on each issue for him to take back to Reagan.