A ransom note appeared today in the case of missing sportsman Tomas Manotoc, a purported son-in-law of the Philippine president. But the Manotoc family, which has alleged official involvement in the apparent kidnaping, denounced the note as a fake.

The handwritten note in the Pilipino language was found early this morning stuck on the fence of the Manotoc family residence in a posh Manila suburb.

Manotoc, 33, a professional basketball coach and top amateur golfer, is said to have failed to return home on Dec. 29 after dinner with Maria Imelda (Imee) Marcos, 26, daughter of President Ferdinand Marcos. The two reportedly were secretly married in Arlington, Va., last month, although a spokesman for the presidential family has denied that the marriage took place.

The note, addressed to Manotoc's parents, listed three demands:

"1. Amnesty for our group; 2. Release Communist leaders Jose Maria Sison, Commander Dante a nom de guerre for Bernabe Buscayno , Fidel Agcaoli and Saturnino Ocampo; 3. Prepare 20 million pesos $2.5 million . Wait for my next letter. Happy New Year." It was signed "Tom."

Manotoc's brother Ricardo Jr., 37, a stockbroker, said the letter was definitely not written by Tommy. "He never signs off as Tom but always Tommy and he always writes in English," he said. "We have specimen handwriting and this is clearly not Tommy's."

The brother also declared, according to United Press International, that presidential security guards came to the Manotoc residence and inquired about the ransom note a full hour before its arrival.

The missing Manotoc was estranged from his first wife, Aurora Pijuan, by whom he has two children. He reportedly had arranged for a Swiss and Vatican annulment of his marriage last month since divorce is not recognized in the predominantly Catholic Philippines.

But in the meantime he had obtained a divorce in the Dominican Republic. Both the Manotoc and Marcos families were said to have been informed only after the marriage, which has caused great displeasure in the two households.

It was the Manotoc family's belief that the couple had presented the marriage as a fait accompli knowing the strong objections.

Ricardo Manotoc Jr. said his family could not afford to pay the ransom nor could it see any connection between Tommy and the leaders of the outlawed Communist Party of the Philippines, mentioned in the note, who are on trial for subversion and rebellion.

A statement issued by the presidential palace last night said its intelligence assessment was that Manotoc was "in the hands of kidnapers who are either subversive or criminal elements."

Manila's evening newspapers have begun publishing stories on the missing sportsman but without linking the incident to the first family.

The Manotoc family resisted official requests yesterday to suppress the news of Manotoc's disappearance for 48 hours. Ricardo Manotoc Jr. said: "We have to move in the direction of what we think is best in Tommy's interest."

He said he has spoken to Imee Marcos on the telephone several times and she seemed to be bearing it well. After the marriage, the couple were not living together in Manila but were meeting discreetly, the Manotoc family said.

Manotoc is the nephew of two anti-Marcos foes in exile in Washington, Raul Manglapus, who heads the Movement for a Free Philippines, and ex-newspaper publisher Eugenio Lopez.