President and Mrs. Carter, surrounded by a swarm of 200 tourists, reporters and cameramen, took a walk down Main Street in Plains yesterday for a folksy reunion with hometown friends.

On the first full day of the Carters' two week vacation, it was a now traditional gesture to the small town from which the Carters came.

It was also one of the few public events scheduled for the trip, most of which has been carefully arranged to give the president maximum privacy.

"We'd like to be by ourselves for a while," the president told reporters earlier in the day.

About 6:30 yesterday morning, the Carters, including the president's mother, Miss Lillian, went fishing at a secluded park on Carter-owned property outside of town. They caught "a basketful" of bass, bluegill and bream in an area marked "No Fishing" by the Carters.

Friday night, they celebrated the birthdays of both the First Lady (her 51st) and the president's mother (her 80th) at a party in nearby Americus, Ga., thrown by the President's brother, Billy.

The Carters will leave Plains tomorrow for a whitewater raft trip down the Middle Fork of the Salmon River in Idaho. Then they will fly to Wyoming for visits to the Grand Tetons and other scenic parks in the area.

While the Carters obliged tourists and the press by posing for pictures, the president said his main purpose here was "to see the people of Plains." The Carters greeted people in the shops with big smiles, affectionate hugs and small talk.

The big question of the day among reporters - who have little real news to report here - was how the president would handle cousin Hugh Carter, whose recent book on the family, "Cousin Beedie and Cousin Hot," has upset some members. Hugh had not been invited to the birthday party the night before, according to Miss Lillian.

The president went to Hugh's Antiques, however, just as he went into all the other shops. Cousin Hugh grabbed the president's hand and pulled him behind the couter for picture taking not far from a table displaying copies of the book, which Hugh sells at the store "personally autographed" for $12.50.

Reporters asked the president if he liked the book. "It was all right," he responded unenthusiastically.

Late yesterday afternoon, the president wearing blue jean cutoffs, sneakers and a baseball hat, pitched a team of Secret Service agents to a 6-5 victory over a local team led by his brother.