The boy, found clinging to a rubber inner tube off Florida after a launch overloaded with Cuban refugees sank, has set off an extraordinary tug-of-love between the nations. His mother and her lover drowned.
Castro demanded the boy - who is six today - be brought back to his home within 72 hours. "If not, there are going to be millions of people in the street demanding the freedom of the boy," he told a Cuban trade delegation returning from Seattle.
Castro complained on state television that the case had provoked an unprecedented "state of irritation"amongst the island's 11 million inhabitants, who believe parents have custody rights. In Florida, Elian has been lodging with an elderly great-uncle and great-aunt who visited Cuba just once.
The row may cause Havana to cancel immigration talks due on 13 December, aimed at implementing agreements signed in 1994. Under their terms, the US would repatriate illegal migrants picked up at sea while Cuba undertakes to stop the flow of departures.
The Cuban leader said keeping Elian in Florida was a breach of the migration accords, which could have serious consequences. "We will not negotiate this. You do not negotiate life, or the right of a father to his son,'' Castro added, saying it was ''a colossal stupidity'' to keep the boy in Florida.