The allegations concern Fr Marcial Maciel (82), Mexican-born founder of the Legionaries of Christ, and date back to the order's creation in 1941.
They span two decades of alleged abuse against about 30 seminarians, who were under 16 at the time.
Despite accusations by nine former members of the order, including letters to the Pope and a formal canon law complaint seeking Fr Maciel's excommunication, the Vatican has remained silent. Fr Maciel has declined to discuss the allegations. After they first surfaced publicly in 1997 he issued a letter saying they were "defamations and falsities with no foundation whatsoever". Although it's not known if the Maciel case will be raised during the Pope's visit, the issue of sexual abuse is likely to be discussed in private talks with Mexican bishops.Legion spokesmen in Mexico and the US say a Vatican investigation at the time of the 1997 accusations cleared Fr Maciel.
But the nine accusers say it failed to properly examine the case and that they went public only after exhausting internal Church channels. The men, now in their 60s, include two university professors, a lawyer, the president of the legion's US branch and the order's one-time treasurer.
Juan Vaca, the legion's former US president, said his abuse by Fr Maciel began at age 10. The accusers' stories were buried for years by Mexico's media, which feared offending the order, which has close ties to the country's political and business elite. The men say Fr Maciel led a double life, displaying strict religious devotion during the day and taking boys, sometimes two at a time, to bed in the evenings.
The legion is Mexico's fastest-growing order, with 503 priests and 2,500 seminarians in 20 countries, including a strong presence in the US, Spain and Ireland. Its influence stems largely from the elite lay movement Regnum Christi, with 50,000 members, including some of Mexico's wealthiest businessmen. President Fox is said to be a member.
Fr Maciel is highly regarded in Rome, where the order has its HQ. He was involved in organising the Pope's four previous visits to Mexico and is expected to accompany the Pontiff, who has repeatedly praised his loyalty to the Vatican and being an "efficacious guide to youth."