DALLAS- Dallas will observe the 50th anniversary of President John F. Kennedy's assassination on Friday with its first official ceremony to mark the event seen as the darkest day in the city's history.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, which starts at 11:30 a.m. (1730 GMT), is set to coincide with the time of day Kennedy's motorcade 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 Nov. 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.After thousands of books, news articles, TV shows, movies and documentaries on that fateful day in Dallas, 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.