The fight over who may stand in this small section of downtown Dallas on Friday has come to symbolise the decades-long friction between authorities and so-called truthers

Until this week, X marked the spots. But on Tuesday a steamroller trundled up and down the road near the building best known as the Texas school book depository, smoothing out a brand-new surface with its smart black asphalt and freshly painted straight white lines.

City workers are re-laying the street. "I guess they don't want any special visitors to see the Xs," said Robert Groden as he sat on the grassy knoll at a table stacked with copies of his DVD, The Case For Conspiracy.

The New Yorker was a photographic consultant to the House select committee on assassinations in the mid-1970s and also provided input to Oliver Stone's 1991 movie, JFK. He moved to Dallas two decades ago and comes to Dealey Plaza most days, selling his DVDs and books.

He claims responsibility for taping white crosses along Elm Street that for years tempted camera-wielding tourists out into the road, dodging traffic on the three-lane artery so they could say they were there, standing in the path of the bullet(s) that changed the world.

Facebook Twitter Pinterest An X marks the spot on Elm Street where the first bullet hit President John F Kennedy. The Xs on the street are now gone. Photograph: LM Otero /AP

Facebook Twitter Pinterest A view of Elm Street and Dealey Plaza from the seventh floor window, directly above the window where Lee Harvey Oswald shot JFK. Photograph: Larry W Smith/EPA

Friday ought to be one of the most important days of his life. It is the 50th anniversary of John F Kennedy's death, as well as Groden's 68th birthday. The city is hosting a commemoration in Dealey Plaza for dignitaries and 5,000 members of the public who won their tickets in a ballot. Groden said he will be there as an accredited journalist. But like other conspiracy theorists, he does not feel welcome.

"It's very late in the day for this damn city to all of a sudden pretend that they care about what happened to the president. For 49 years people like myself have been out here trying to keep the issues alive, respectfully giving a moment of silence for the president and now that it's the magic 50th, the city decides they want to take it over," he said.

Dallas civic leaders have arranged an hour-long ceremony called "The 50th" that will "set a solemn, dignified and understated tone as we commemorate the life, legacy and leadership of President John F Kennedy," mayor Mike Rawlings said in a statement. There will be speeches, hymns, a flypast and a moment of silence at 12.30pm, the time of the shooting.

Attendees have been background-checked by Dallas police. Security will be tight, dissenters and demonstrators are unwelcome. With the world's media in attendance, for the theorists this is akin to being asphyxiated within touching distance of an oxygen tank.

The fight over who may stand in this small section of downtown Dallas on Friday has come to symbolise the decades-long friction between authorities and truthers.

Understandably, after half a century, many in Dallas would like this anniversary to be an orderly and tasteful form of closure, the point when history was finally left in the past. The conspiracy theorists want precisely the opposite: for the attention to spark more debate, greater openness, extra focus on whatever they happen to believe.

In the absence of any previous official city event, November 22 has always been their day, the plaza their place. The city "had Dealey Plaza all these years and they didn't take advantage of it," said Debra Conway, co-founder of JFK Lancer, a historical research company that believes there was more than one shooter. Conway is organising a conference for 400 people in Dallas this week. She will watch the ceremony on television.

"They're just on lockdown, they don't want us there and I have no desire to get arrested," she said. "They're not acknowledging that somebody died there."

Facebook Twitter Pinterest Obama and Clinton families lay a wreath at the gravesite for President John F Kennedy at Arlington National Cemetery. Photograph: UPI /Landov /Barcroft Media

The Washington-based Coalition on Political Assassinations (COPA) is also holding a conference in Dallas and expects 250 attendees. It considered filing a lawsuit seeking to compel the city to grant its members access to the plaza but reached a compromise with organisers last week. They will gather at a nearby parking lot, hold a period of silence at 12:30pm then be escorted by police into the plaza two hours later, long after the official ceremony is over.

Some will wear T-shirts bearing the conference logo: a coin with a depiction of a bullet hole dripping blood from Kennedy's head and the slogan "50 years in denial is enough. Free the files. Find the truth."

John Judge, COPA's executive director, describes the group as at the "serious end of the research community". They believe they are investigating an unsolved murder. "Most of us feel that Oswald is not responsible, he was a patsy," Judge said. He feels the restrictions are "a denial of free speech", the ceremony a whitewash: "this is a PR event, they know the press is coming at that moment to that place and they want to control the message."

Workers erected barriers around the centre of the plaza and began setting up seats and a stage on Tuesday. About a dozen police officers kept watch while street vendors tried to hawk pamphlets to passers-by and trolley tours of JFK-related sites left from a bus stop next to the Depository, which houses the Sixth Floor Museum.

Groden's works are not for sale in its bookshop. While the museum is thorough and does not shy away from acknowledging some of the most popular conspiracy theories, Groden sees it as a tool of official suppression: " I come down here because people want to know the truth, they come to Dallas to learn the truth and they're not getting it from the Sixth Floor Museum. There needs to be somebody here with a reasonable sense of what actually did happen." He thinks the evidence indicates there was more than one shooter.

To the suggestion that after so many years of speculation and innuendo, pro-conspiracy theorists as a whole have been marginalised, discredited and ridiculed by the mainstream, researchers such as Judge have a simple riposte: they are the mainstream. Judge cited a 2012 poll by the History Channel which found that the vast majority of respondents did not trust the Warren Commission's lone-gunman conclusion.

An Associated Press poll conducted in April this year concluded that 59% of Americans think multiple people were involved in a conspiracy to kill Kennedy.

"We've won the debate. Most people want what we're calling for, all the files released, a further investigation. It's an important year for us, a chance to be visible," said Judge.

"Our history has basically been stolen from us by this secrecy. I think the military-industrial complex had a great deal to do with the Kennedy assassination. I don't think a hoopla event gives you a catharsis. I don't know how you celebrate the life of JFK and refuse to solve his murder."

Facebook Twitter Pinterest Plaque on the Dallas county administration building signifies the site where Lee Harvey Oswald shot JFK from a sixth-floor window. Photo: UPI /Landov /Barcroft

A large Texas Historical Commission plaque is fixed to a corner of the Depository, which is properly called the Dallas County Administration Building. The first 150 words explain its prosaic origins. But the final paragraph is more intriguing: "On November 22, 1963, the building gained national notoriety when Lee Harvey Oswald allegedly shot and killed President John F Kennedy from a sixth floor window as the presidential motorcade passed the site."

A sharp object has been used to scratch the black surface of the metal, underscoring "allegedly" with an uneven white line.