Many of the 400 signers of the open letter are from Latin America, and numerous Nobel Peace laureates are listed, such as former Archbishop Desmond Tutu of South Africa and activist Rigoberta Menchu of Guatemala.
Announcing the letter at a news conference, leading Cuban writer Roberto Fernandez Retamar said Cubans are convinced that Castro's handover of power to his younger brother and defence minister, Raul Castro, is only temporary.
That optimistic assessment has been reinforced by statements from Fidel Castro's inner circle and Latin American allies, who say the Cuban leader is recovering well from surgery for internal bleeding.
Cubans were told most details of his health would be kept "a state secret" to prevent enemies from taking advantage of his condition. Indeed, officials haven't said precisely what ails Castro or what surgical procedure he underwent.
U.S. President George W. Bush said the United States remains in the dark about the illness, but he didn't miss the chance to motivate anti-Castro activists to push for change, according to the AP.
On Sunday, Secretary of State Condoleezza Rice denied the United States is contemplating an invasion of the island in the wake of Castro's illness but said the U.S. wants to help Cubans prepare for democracy.
The Castro brothers have been out of sight since the July 31 announcement on state television that Fidel had undergone surgery and was temporarily ceding power to Raul. Meanwhile, the government has tried to promote a sense of normalcy, an image Retamar fostered Monday.
A growing number of Cuban officials and Castro allies have come forward, meanwhile, to say he will return soon.