HAVANA (Xinhua) -- More than 400 intellectuals from 50 countries called on the United States in a signed open letter to stop threatening Cuba and to respect the nation's sovereignty, some of the signatories told a press conference here on Monday.

"Faced with the growing threat to one nation's integrity, and to the peace and security of Latin America and the world, we the undersigned demand that the United States government respect Cuba's sovereignty," says the letter that was jointly presented by Belgian sociologist and theologian Francois Houtart, editor of themagazine Areito, Andres Gomez, and president of the Casa de las Americas, Roberto Fernandez Retamar.

The letter denounced statements made by U.S. government officials following the July 31 operation on Cuba's leader Fidel Castro to control an intestinal hemorrhage. U.S. President George W. Bush threatened after Castro's illness that "we are working actively for a change in Cuba, not simply hoping it will happen," while U.S. trade secretary Carlos Gutierrez's said, "The moment has come for a transition to a real democracy."

Castro, who turns 80 on Aug. 13, has been out of sight since July 31, when his secretary went on state television to announce he had undergone surgery and was temporarily ceding power to Defense Minister Raul Castro. The intellectuals also denounced a recent leaked plan by the Commission for a Free Cuba, which is run by Condoleezza Rice, the U.S. secretary of state. The plan included anti-Castro measures that "must remain secret for reasons of national security and so they can be realized effectively."

The letter said it was not difficult to imagine what such measures might be, adding that "we must halt a new aggression at all costs."

Cuba has been under a U.S. financial embargo since 1960s. Castro, who took control of Cuba in 1959, has survived repeated U.S. attempts to topple him.

Fernandez, president of Casa de las Americas, called on intellectuals of the world to sign the letter on internet.

Signatories included Nobel Prize-winning writers Jose Saramago from Portugal, Wole Soyinka from Nigeria, Adolfo Perez Esquivel from Argentina and Dario Fo from Italy. Social activists Desmond Tutu and Nadine Gordimer from South Africa, and Rigoberta Menchu from Guatemala, also put their names to the letter.