HAVANA (CNN) -- In what has become a daily occurrence, hundreds
                  of thousands of Cubans marched Thursday to again demand the
                  immediate return of 6-year-old shipwreck survivor Elian Gonzalez.

                  Cuban President Fidel Castro has called for the massive street protests to
                  press the United States into returning the boy immediately to his father in
                  Cuba. An estimated 300,000 people turned out for Thursday's
                  demonstrations.

                  Elian was found clinging to an inner tube off the Florida coast November 25
                  after a boat carrying a group of migrants including his mother and stepfather
                  capsized. His mother, stepfather and eight other Cubans apparently
                  drowned. Elian and two others were rescued.

                  U.S. authorities handed Elian over to his great-aunt and great-uncle in
                  Miami. They want the boy to stay in the United States.

                  In Washington, Deputy Attorney General Eric Holder said Thursday that a letter had been
                  sent to Elian's father regarding the boy's possible return to Cuba.

                 Immigration officials would make final decision

                  The five-paragraph note to Elian's father, Juan Miguel Gonzalez, from INS District Director
                  Robert Wallis outlines the various documents the elder Gonzalez must present to INS
                  officials to establish parentage. Officials said the letter was being hand delivered Thursday.

                  Holder said the father's relationship with his son will be a key factor in the final decision.
                  The father has told reporters that the boy lived with him and that he has Elian's birth records.
                  He has said that the boy was taken out of Cuba without his knowledge.

                  Holder also said the final decision on whether the boy will stay in the United States or go
                  back to Cuba will likely be made by the U.S. Immigration and Naturalization Service.

                  "I don't think we anticipate that it would go to state courts at this time," Holder said.

                  Castro has been pressing for the boy's return, citing U.S.- Cuban immigration
                  accords agreed to by the two countries.

                  Under those regulations, immigrants who make it to the United States are allowed to stay.
                  Those who are picked up at sea, as was Elian, are normally returned to Cuba.

                  There have been large demonstrations for days in front of the American
                  Interests Section in Havana seeking the boy's return. The Interests Section
                  is surrounded on all sides by Cuban flags, pro-government propaganda and
                  photos of Elian.

                  With emotions running high in the case, the street protests have become a
                  surreal mixture of party atmosphere and denunciations of U.S. society.
                  Popular singers and local sports stars have attended, entertaining the crowds
                  amid speeches slamming the "imperialist Yankees."

                  Washington's senior diplomat in Havana, Vicki Huddleston, has formally told
                  Cuban officials that U.S. immigration authorities are ready to meet with
                  Elian's father, anywhere he wants -- including Miami.

                 Hijack suspects returned to island

                   In a separate development, the U.S. Coast Guard brought back to Cuba
                   six alleged hijackers and two tourism workers whose boat they took at
                   knifepoint in another illegal immigration attempt earlier this week.

                   The two Cuban workers were injured during the incident.

                  One of them, Carlos Alberto Hernandez, was taken off the boat on
                  a stretcher and put into an ambulance after the hand over Thursday at
                  Cabanas Bay, some 40 miles west of Havana.

                  Cuban sources said the alleged kidnappers would be immediately tried and
                  could face sentences of up to life imprisonment.

                  Havana had demanded the return of the suspects and applauded the U.S.
                  decision to send them back.

                 Aid agency says Cuba disallows flight

                  The aid agency AmeriCares said Thursday the Cuban government had withdrawn
                  permission for a shipment of medical supplies to be flown to Havana on
                  December 11.

                  Drew Hannah, vice president of AmeriCares, said no reason was given.

                  However, Enrique Comendeiro, a spokesman for the Cuban Ministry of
                  Health, said the problem had to do with scheduling. He said the ministry wants the
                  donations but wants the plane to come to Cuba at a more "appropriate time."

                  He did not elaborate.

                  The shipment of $3.4 million worth of medicine and other supplies was
                  scheduled to leave Fort Lauderdale, Florida, on Saturday but was canceled
                  suddenly after AmeriCares received word from the Cuban government that
                  visas and flight permission had been denied.

                  In addition to medical supplies, the shipment was to carry a large number of
                  Christmas toys for distribution in Cuba.

                       Terry Frieden, The Associated Press and Reuters contributed to this report.