The rejection had been expected in large part because the group from the Inter-American Commission on Human Rights had been invited by the country's congress. The legislature and Maduro mutually consider one another illegitimate.
The three members of the group posted a picture on Twitter holding their boarding passes at the airport gate in Panama. They said Maduro's government told the Panama-based Copa Airlines not to allow them on the flight to Caracas.
"We will press ahead with our objective, meeting victims on the Colombian border with Venezuela," said Paulo Abrao, executive secretary of the commission.
Venezuela's opposition-led National Assembly had invited the group, which is a branch of the Organization of American States, the Washington-based body representing more than 30 nations in the Western Hemisphere, whose leaders have been among the fiercest critics of Maduro.
Maduro's Foreign Minister Jorge Arreaza on tweeted on Friday that they would be blocked.
"Venezuela has not invited or accepted a delegation visit from the IACHR," Arreaza said, adding that the visit "is not authorized."
Maduro's government in 2017 started a two-year process to leave the OAS, saying that it is a pawn of the United States.
The OAS itself has joined the U.S. and more than 50 other nations in withdrawing recognition of Maduro on grounds his reelection in 2018 was illegitimate. It recognized congressional leader Juan Guaidó as acting president.
Maduro, however, controls all the mechanisms of government, including the military and courts, and he recognizes an alternative version of the National Assembly led by a minority of lawmakers allied with him.