“It’s absurd to move the discussion of his death to another moment,” said John Judge, executive director of the Coalition on Political Assassinations, a Washington, D.C., nonprofit that studies 1960s murders of public figures. “Our First Amendment rights are being violated.”

Mr. Judge, 65 years old, said conspiracy-theory proponents have gathered at Dealey Plaza every Nov. 22 since 1964. Next year, he added, will be the first that Dallas hasn’t granted a permit for the meeting, which usually involves a moment of silence and a few speeches. He said the city should move its ceremony elsewhere, adding that his group’s members would find a way to disseminate their theories during the city event, possibly even dropping protest banners from nearby buildings.

Mayor Mike Rawlings said in an interview that he would meet with Mr. Judge’s group, as well as with others who object to the city’s plans, to hear their concerns. But he is determined to keep the tone of the event reflective of the “international, cosmopolitan, arts-centered city” Dallas is today, he said, while focusing on President Kennedy’s life and accomplishments. “For 40 minutes, we need to be focusing on the man, not the moment 50 years ago,” Mr. Rawlings said.