The big stars are all lined up for Monday night's fund-raising premiere of "Samaritan: The Mitch Snyder Story" at the National Theatre, but the ticket sales are not good. As of late yesterday, only 250 seats had been sold for the advance screening of the CBS television movie starring Martin Sheen and Cicely Tyson. And the National has a seating capacity of more than 1,670.

Mitch Snyder said yesterday there hasn't been enough time to organize this joint benefit for the Community for Creative Non-Violence shelter and for Love Is Feeding Everyone, which was organized only a few weeks ago. As late as Tuesday evening, about 2,000 invitations went into the mail for the $50-a-seat screening, or $250 a seat for the film and party afterward in a tent on the plaza across from the theater. Among the promised celebrities for the gala are Sheen, Valerie Harper, Dennis Weaver, Lynda Carter, Esther Rolle, Maud Adams, Susan Sullivan, John Savage, Dick Gregory, Polly Bergen, Dr. Benjamin Spock and Eunice Shriver.

Snyder said he and other CCNV volunteers are manning the telephones trying to sell tickets and to get corporate sponsors. He said he's not worried even though a Los Angeles showing last Thursday wasn't well attended. Snyder explained that one of the movies that had a great impact on him was "It's a Wonderful Life." He said the traditional Christmas film starring Jimmy Stewart shows that "if we do good and try to do the right thing when the chips are down, the community will be there to help and create that kind of miracle."

On the Campaign Trail

One of the original Freedom Riders, John Lewis, is in town this week drumming up support for his campaign for Congress from Atlanta's 5th Congressional District in a campaign that pits him against five other candidates, including Georgia state legislator Julian Bond. Lewis, who is a member of Atlanta's City Council, remembered that 25 years ago this past Sunday, he and seven whites and five other blacks started their famous Freedom Ride from Washington into the South.

They rode public buses and were testing public facilities such as bus stations, restrooms and lunch counters. He remembers that the first violence broke out in Rock Hill, S.C., and that in Anniston, Ala., the bus they were riding in was firebombed. Lewis feels his major opponent in the Democratic primary Aug. 12 is Bond, who, like Lewis, was in town a few weeks ago looking for political and financial support.

End Notes

Television stars Richard Chamberlain and Jaclyn Smith have signed on to host the annual Ford's Theatre all-star salute. The black-tie gala June 8 will be taped for showing June 25 on CBS. Last year, President Reagan and Nancy Reagan attended the show at Ford's that brought in about $500,000. It was also the evening that an uncomfortable president and House Speaker Tip O'Neill complained about the theater chairs, and new chairs have been installed. This year's gala will have the same general chairman as last year, Mary Jane Wick, and the same cochairmen, Carol Laxalt and Millie O'Neill . . .

That Mayor Clint Eastwood knows how to keep campaign promises like a true politician. At his first city council meeting Tuesday he brought the sale of ice cream cones back to Carmel-by-the-Sea, Calif. A strict ordinance on water use and takeout food had virtually prevented ice cream shops from opening. It was a campaign issue. "I hope I never again have to read about ice cream cones," he said as he moved on to other important community problems, like public toilets and parking. He received a congratulatory call earlier in the day from Nancy Reagan's staff asking him to play in the May 24 White House tennis tournament that raises money for Mrs. Reagan's Drug Abuse Fund. Eastwood hasn't decided yet about the invitation . . .

Royal Watch: Just when it was all settled that Princess Diana was fine and not pregnant, Prince Charles had to have his little joke. At a farewell dinner Tuesday in Vancouver, where they had been visiting Expo 86 -- the royal couple left yesterday for a five-day visit to Japan -- he said of her fainting spell: "It is really due to the extremely advantageous conditions that pertain in British Columbia -- the weather and the general fertile conditions, which have ensured she is about to have sextuplets, which is really why she fainted." Then he added quickly, "It's not actually true." That should keep the London tabloids speculating for weeks.