Dealey Plaza in Dallas is seen from the roailroad trestle atop the triple underpass looking towards the spot where the President John F. Kennedy was shot in 1963, as people gather at the site to commemorate the 50th anniversary in Dallas, November 21, 2013. [Photo/Agencies]

DALLAS - Dallas will observe the 50th anniversary of President John F. Kennedy's assassination with its first official memorial on Friday, as the rest of the nation pauses to remember the event that changed history.

At Arlington National Cemetery in Virginia where Kennedy is buried, family members laid a wreath at his grave, where Jackie Kennedy and two of their children also are buried.

At dawn, Attorney General Eric Holder made a gravesite visit to honor Kennedy, bowing his head and placing a Justice Department commemorative coin at the stone. Holder then walked a short path to the grave of Robert F. Kennedy, who had served as attorney general under his brother, bowed his head and left another coin.

In a late morning ceremony in Dallas, Kennedy will be remembered with prayers, a speech by Mayor Mike Rawlings and military jets flying over the city's Dealey Plaza, where Kennedy was shot.

The ceremony starts at 11:30 a.m. (1730 GMT), to coincide with the time that Kennedy's motorcade had passed through packed downtown streets, 50 years ago. Only 5,000 people will be able to view ceremonies in Dealey Plaza but video screens will be set up throughout downtown.

For previous anniversaries, conspiracy theorists who say there was a plot to kill Kennedy have usually taken over Dealey Plaza, denouncing the official line that Lee Harvey Oswald acted alone and fired three shots at Kennedy from the sixth floor of the Texas School Book Depository building.

They have come to Dallas in force this year but will not be part of the official event.

"His death forever changed our city, as well as the world," Rawlings said in a statement ahead of the anniversary. "We want to mark this tragic day by remembering a great president with the sense of dignity and history he deserves."

Dallas was seen as a pariah city for years after the November 22, 1963, assassination, and avoided any commemoration. That stigma started to fade decades ago, and now, The Sixth Floor Museum in the former Texas School Book Depository is one of the city's largest tourists attractions.

"Dallas came under a great deal of international criticism after the assassination. It was called the 'City of Hate,'" said Stephen Fagin, associate curator The Sixth Floor Museum.

Amid the Cold War paranoia and simmering racial tension of the 1960s, a small but influential group of arch-conservatives protested Kennedy's visit to Texas, saying he was soft on communism and should stay away.

In recent days, the city removed a large "X" embedded into the pavement by an unknown person or people that marked the spot on Elm Street where Kennedy was shot in the head.

The "X" had been seen as tasteless by many while the official observance - a small plaque on the plaza's noted "grassy knoll" - had been criticized as inadequate.

Thousands of books, news articles, TV shows, movies and documentaries have been produced about that fateful day in Dallas, and surveys show a majority of Americans still believe in the conspiracy theories, distrusting evidence pointing to Oswald as the sole killer.

Hugh Ayensworth, a reporter in Dealey Plaza 50 years ago who witnessed the assassination and also saw Oswald shot dead by Dallas nightclub owner Jack Ruby, has spent a lifetime investigating the killings and debunking suspected plots.

"We can't accept very comfortably that two nobodies, two nothings - Lee Harvey Oswald and Jack Ruby - were able to change the course of world history," he told Reuters.