Barcelona forward Lionel Messi has been criticised by the Human Rights Foundation (HRF) for showing "enthusiastic support for the dictatorship" during his trip to Gabon last month.
Messi, 28, visited Gabon, which is set to stage the 2017 African Nations Cup, and met President Ali Bongo Ondimba, who drove him around the capital, Libreville.
HRF president Thor Halvorssen said on his organisation's website: "In providing PR services to Gabon's Bongo family, Lionel Messi has seriously undermined the credibility of his own charitable foundation.
"Whereas Messi claims to support children's rights, and even serves as a UNICEF ambassador to promote youth education, he has endorsed a kleptocratic regime that refuses to investigate the ritual murder of children in Gabon."
He added: "Messi's trip is part of the Bongo family's PR campaign to promote the 2017 Africa Cup of Nations, which Gabon will host at enormous expense despite the fact that the Bongo family's embezzlement has left 20 percent of the population to live on less than $2 per day."
The Argentina international laid the first stone in the building of the Port-Gentil stadium, which is to be used at the African Nations Cup, and is also reported to have appeared at one of the president's restaurants in the city.
El Mundo Deportivo has reported that, contrary to a claim in France Football, Messi was not paid for the trip.
Bongo had previously explained the visit, saying: "When I was in Barcelona a few years ago, I met Messi, who had told me that he would come to visit me in Libreville. It's a promise he made me. He is a man of honour who just kept his word."
HRF chief strategy officer Alex Gladstein cited the examples of several other celebrities who had made controversial foreign visits in recent years, including Jennifer Lopez singing for the leader of Turkmenistan, Mariah Carey holding a concert for Angola's president and Kanye West performing at the wedding of the Kazakhstani president's grandson.
"It's shocking, given the recent public humiliation endured by Jennifer Lopez, Mariah Carey, and Kanye West, that Messi's management and the athlete himself would consider putting his prestige at the service of a serial human rights abuser," Gladstein said.
"If Messi wants to do good in Gabon, HRF recommends that rather than partying with oppressors, he release a statement in solidarity with the country's persecuted dissidents and environmentalists, and press the regime to investigate the ritual murders of children."
Messi had previously faced criticised for wearing denim shorts and a T-shirt during the visit to Gabon, with the country's political opposition -- the Union du Peuple Gabonais (UPG) party -- accusing him of dressing "like he were going to a zoo."