More than 500 families, joined by similar doubt, anger and pain, gathered on the 10th anniversary of the Paris Peace Accords yesterday for two days of wreath-laying, candlelight vigils, briefings, speeches, reunions and memories of a war that--for them--goes on.

The special meeting of the organization of families of men who were held prisoner or listed as missing in Vietnam marked not just the anniversary of the agreement that ended the nation's longest war, but a new chapter in the effort to resolve the fate of 2,494 Americans still unaccounted for in Indochina.

"It's kind of a show of strength," said Patti Sheridan, a spokesman for the National League of Families of American Prisoners and Missing in Southeast Asia. "The issue [of the unaccounted-for soldiers] is back on the front burner, where it should have been for years. Now it is no longer an embarrassment to talk about it."

The relatives, most of them league members, were flown here courtesy of the Air Force on regularly scheduled military flights, arriving at Andrews Air Force Base, and then taken to the Hyatt Hotel in Crystal City. They wore name tags that told their relationship to a soldier: sister, son, father, mother.

The stories on their lips were of last visits and what had happened in the years since.

There were tears and patriotism in abundance at a luncheon when a Navy chaplain said, "Bind us together with the perserverance our sons and husbands took into Southeast Asia so long ago," and the Marine Drum and Bugle Corps played "When Johnny Comes Marching Home." And there was applause for the head table richly stocked with congressmen, generals and service secretaries.

President Reagan, who is scheduled to address the group this morning, sent national security adviser William P. Clark to speak to the gathering yesterday. Clark said that Vietnam, by failing to act quickly to help resolve the issue of the unaccounted for soldiers, has risked "engendering a perception of incredible cruelty."

"Continued violation of the most fundamental moral principles," Clark said, "can only lead [Vietnam] further into international isolation."

In addition to hard-hitting words from high-ranking officials, the National League of Families and the government are seeking to bring the pressure of the American people to bear on the issue, and have launched a nationwide publicity campaign to drum up support.

Television monitors in the lobby offered previews of public service spots that have been sent to more than 400 television stations. One series, produced by the Defense Department, showed pictures of people grieving at the Vietnam Memorial as the voiceover said, "It's been 10 years since the Paris Peace Accords and some 2,500 men are still unaccounted for." Another series, produced by a Los Angeles entertainer-performer, Fred Travalena, featured Bob Hope and a bevy of lesser stars urging viewers to "ask for an accounting."

For some, the meeting was a notably emotional occasion. One woman, whose husband's status had been changed from Missing in Action to Killed in Action, shouted in to a microphone: "I am the wife of an MIA. My husband was declared KIA. I want him MIA! I waited for him and I still do. I still wait for him to come home!"

Jackie Benson of Hot Springs, Ark., pointed to the silver bracelet engraved with the name of her brother, Air Force Maj. Charles E. Morgan, who was presumed dead after he was shot down over North Vietnam in 1966 but whose body was never recovered.

"I still wear my bracelet," she said. "I'm here because I want something back before I take it off."

In front of Benson sat Constance Hestle, whose husband, Col. Roosevelt Hestle, had been Morgan's pilot.

The two women exchanged letters in 1977, but had never seen each other until Wednesday, when they met on a military plane.

"When we got to Louisiana, I was getting ready to sit down," said Hestle. "Jackie said, 'Are you Connie?' And I just knew it was her. It was fantastic. Arms and plates were going everywhere."

An MIA wife who had remarried brought her 13-year-old son A.J., who was born too late to know his father, Thomas W. Underwood, a marine sergeant killed in a helicopter crash over Laos in 1970.

"His father died when A.J. was a baby," said Ina Kay Zimmerman, who, at 41, is about to open a law practice in Olathe, Kan.

"It was so sad. But we've explained that you can be a father in two ways. When I think about that part of my life, it's like I'm watching an old movie. The knife still sticks in me, but the pain has been dulled.

"I began my education after my husband died. I've gotten much more sophisticated . . . . I had strength, but it was more endurance. It said, 'I will just keep putting one foot in front of another . . . . I will endure.' "