The public, the experts, and the special interests had their say yesterday in a five-hour hearing on how they feel about the Food and Drug Administeration's proposal to ban saccharin from diet beverages and foods.

One parent, K. David Stone, of Kansas City, representing the Heart of American Affiliate of the American Diabetes Association, said his 15-year-old daughter Jennifer needs saccharinsweetened sodas not only to control her weight, but also to lead a normal social life.

Similarly, two women testified that only with saccharin were they able to hold down their weight and maintain their health.

But Dr. Sidney M. Wolfe, director of Ralph Nader's Health Research Group, said there is "no scientific evidence" showing saccharin to be important for weight control.

He cited a three-year study showing "no significant difference in weight loss between users and nonusers" of artificial sweeteners, of which saccharin is the only one now commercially available. And, Wolfe said, saccharin stimulates appetite in certain obese laboratory animals.

The witnesses and spectators - more than 125 of them - included at least 30 who came from Atlanta on "The Saccharin Special Train" to the hearing in the Department of Health, Education, and Welfare auditorium, where they held up placards in front of television cameras. One sign read, "Take Away Saccharin/Take Away Life."

Rep. Abner J. Mikva (D-Ill.), usually one of the FDA's staunchest allies, joined other critics of the ban who expressed incredulity that the government proposes to end the use of saccharin while not proposing to prohibit cigarettes. Congress has commanded the FDA to assure the safety of food while denying it power to do anything about smoking.

The hearing, which resumes today, produced to "blinding flashes" of new information, FDA Commissioner Donald Kennedy told reporters. But it sharpened conflicts over some fundamental issues. Perhaps the most volatile one is the Delaney Amendment, a 1953 law that requires the FDA to keep out of foods any additive shown by sound tests in laboratory animals to cause cancer.

For years, food processers and some scientists and legislators have urged repeal or dilution of the amendment, contending that the FDA should be allowed to assess the benefits and risks of each suspect additive.

Milkva termed the amendment "nonsensical" - "a Pavlovian response to a Pavlovian test." But FDA official told reporters privately that, by making decisions on cancer-causing additives subject to administrative discretion, changes in the amendment could expose them to heavy political pressures.

Mikva is one of 194 House sponsors of a bill to modify the Delancey weigh benefits against risks in dealing with food additives that cause cancer in laboratory animals.

Backed by Rep. James G. Martin (R-N.C.), the principal sponsor, Mikva appealed to Kennedy to postpone the probable effective date of the ban-late summer - so that Congress can legislate "without a gun at its head." Kennedy indicated to reporters that a delay is unlikely.

Martin testified that the amendment gave the FDA "no choice" but to ban saccharin once its scientists were satisfied that Canadian government tests established that it caused bladder cancer in rats. "You must ban it at the drop of a rat," Martin told the commissioner.

But Kennedy emphasized that if the Delaney Amendment never had been enacted, he, at least, would have proposed the ban because the basic food-additives law requires positive findings of safety.

No one has provided such findings for saccharin. Instead, some critics of the agency want it to carry a burden Congress never imposed: proving that a food additive, sometimes in combination with other carcinogens, may cause cancer in humans. Such proof is difficult partly because the latent period before detection of cancer may be as long as 40 years.

Saccharin has been in an officially suspect category since 1972, on the basis of animal tests incriminating it in bladder cancer. Terming the tests inconclusive, the FDA did not propose the ban until last March, when the Canadian government completed its studies on 200 rats. Half had been exposed to saccharin from the time of conception. Twenty-one of the animals developed bladder cancer, compared with one of 100 untreated control rats.

While proposing to ban saccharin from processed foods and baverages, the FDA intends to allow "table top" use by classifying it as a nonprescription drug labeled for persons on a sugar-restricted diet. The label would warn that saccharin many increase the user's risk of cancer.

At the same time, the FDA seeks to phase out saccharin as a sweetener of toothpaste, mouthwash and other ingestible cosmetics, and of prescription drugs.

This proposal drew heavy fire yesterday from the American Dental Association, which said it could set back dental health by, among other things, removing an effective but innocuous lure to tooth-brushing. The Pharmaceutical Manufacturers Association said that some of the more than 619 medicines sweetened with saccharin would be so bitter without it that some patients, including children, couldn't swallow them.