Undated (AP) _ With parades, prayers and the playing of taps, Americans honored their war dead Monday on a Memorial Day made freshly somber by the painful memory of the 37 sailors killed on the USS Stark.

For many, like the 300,000 expected on South Carolina's Grand Strand beaches, the day was also the start of swimming pool and beach season. But high in the Colorado Rockies, Trail Ridge Road through Rocky Mountain National Park was closed by snow.

And in Philadelphia, a summerlong party opened for a 200-year-old document called the U.S. Constitution.

Memorial Day ceremonies and services were held in communities across the nation, from solemn observances with 21-gun salutes and the playing of taps to small-town parades and speeches and dedication of new Vietnam veterans' memorials.

Some of those community observances were dedicated to home-town boys killed aboard the Stark in the Persian Gulf in an Iraqi missile attack.

At the USS Arizona Memorial in Pearl Harbor, military representatives were to present a lei containing 2,335 flowers, one for each of the men killed in the Japanese attack on Dec. 7, 1941.

Cannons were fired 21 times at one-minute intervals beginning at noon from the decks of the USS Constitution, the Navy's oldest commissioned warship.

At Fort Campbell, Ky., a black granite monument was unveiled to honor the 248 soldiers of the 101st Airborne Division who died in a 1985 plane crash at Gander, Newfoundland, while flying home from a peacekeeping mission in the Middle East. ''A season of grief has come to a close,'' said Maj. Gen. Teddy Allen, the base commander.

In many communities, volunteers placed flags on soldiers' graves.

''All of the men we recognize here today with our little flags, they saved America for us,'' 70-year-old Helen Witt said at a cemetery at Fort Worth, Texas. ''God forbid that we ever forget why we do these things on Memorial Day, that these men laid down their lives and saw their comrades die.''

At Dover, Del., site of the Air Force mortuary, preparations were made for Tuesday's arrival of an Air Force plane carrying the remains of 36 of the Stark victims. The body of one sailor was not recovered.

''I hope we can one day rededicate ourselves, not in the memory of the 37 men and their families who grieve for them today, but for the cause of world peace,'' Massachusetts Gov. Michael S. Dukakis said after marching in the annual Memorial Day parade in Brookline, Mass., his hometown.

In a ceremony at the Presidio National Cemetery in San Francisco, Col. D. Peter Gleichenhaus compared the lives of American soldiers to chips in a poker game.

''Those of you who are here today on active duty all took the opportunity to ante up,'' said Gleichenhaus. ''There are others who we remember today who made that commitment and whose chips were collected.''

In Auburn, N.Y., World War II veteran Tom King displayed 38 American flags in the front yard of his home for the Stark sailors and his father, a World War I veteran.

''I'm a Navy man myself,'' said King, who served 3 1/2 years in the South Pacific.

Navy Secretary James Webb placed a wreath from President and Mrs. Reagan at the Tomb of the Unknowns in Arlington National Cemetery.

''It is better to spend dollars for readiness than it is to spend lives,'' Webb told people gathered in an amphitheater near the unknown dead from America's last four wars.

At the Vietnam Veterans Memorial in Washington, Sen. Albert Gore Jr., D- Tenn., told a crowd of about 3,000 people that ''the best memorial we can possibly create'' to honor the dead from Southeast Asia would be to ''make war itself obsolete.''

''The next war, if it is a nuclear confrontation, could be the most obscene thing in history,'' said Gore, a Vietnam veteran who is a candidate for the Democratic presidential nomination.

Vice President George Bush and former Chief Justice Warren Burger went to Philadelphia to formally open the observance of the bicentennial of the Constitution. During the weekend, representatives of the 13 original states re-enacted the debate that went into the drafting of the document.

A Memorial Day service for the 273 victims of an American Airlines crash exactly eight years earlier, on May 25, 1978, was held in rainy weather at Chicago's O'Hare Airport.

Former prisoners of war gathered for a small ceremony in a cemetery at Las Cruces, N.M., some of them bound together by memories of the horrors of the Bataan Death March after they were surrendered to the Japanese in the Philippines in World War II.

''It was one hell of a horrible experience and I lived it for a long, long time,'' said Donald Harris.

In Kansas City, Mo., the Rev. Thomas Denzer recalled his service in the Pacific during World War II in a speech at the city's Liberty Memorial.

''It was only when I stood at the graves of my comrades in the steaming jungles of New Guinea and the sticky mud of the Philippines, and I heard the plaintive refrain of taps echoing off the hills that I came to realize that from death and sacrifice comes life and freedom,'' Denzer said.

Americans of Japanese ancestry who served in the U.S. armed forces were honored Monday at a cemetery atop Capitol Hill in Seattle.

In Southern California, where cool weather kept crowds away from beaches, crosses were planted at the Westminster Civic Center for Americans still being held hostage in Lebanon, and Marines in Santa Ana re-enacted the raising of the American flag at Iwo Jima during World War II.

In New York City, ex-servicemen gathered at a Vietnam monument on Memorial Day to back a measure to establish a shelter in the city for homeless veterans.

''On Memorial Day everyone focuses on those veterans who died, but we have to take care of the ones who are dying,'' said John Rowan, a member of the national board of directors of the Vietnam Veterans of America.

In West Virginia, which had the nation's highest casualty rate in the Vietnam War, a Vietnam Veterans Park was dedicated at Philippi by Sen. Jay Rockefeller. ''This park will always remind us of the dangers they face and the sacrifices they endured,'' he said.