Cuban police have increased patrols in parts of Havana, apparently due to uncertainty surrounding President Fidel Castro's health.
Unofficial reports from the Cuban capital also say the government has activated rapid-response brigades trained to handle civil disturbances.
However, foreign reporters in Havana say Mr. Castro's supporters are confident that Cuba's Communist system will remain intact, no matter what happens to the only president most of the country's residents have ever known.
Mr. Castro, who took power in Cuba in 1959 as a young revolutionary leader, stepped aside as president and Communist Party leader on Monday and appointed his brother Raul to replace him temporarily, while he recovers from surgery.
A statement in Mr. Castro's name has said he was being treated for an abdominal bleeding problem caused by stress, but no further details were released.
A senior government official in Havana said the 79-year-old president emerged from surgery feeling very alert.
Mr. Castro's sister Juanita Castro Ruz, who lives in the United States, told reporters she has heard from contacts in Cuba that her brother is "very sick," but that he has now been released from an intensive-care facility.
Neither of the Castro brothers has been seen in public this week.
The large Cuban exile community in Miami, Florida, has rejoiced since Mr. Castro's illness was announced. Many Cuban-Americans say they feel Fidel Castro will not return to power, and that their homeland is about to enter a new political era.
This week's events have raised concerns that Cuban emigres might try to return home abruptly, or that residents of Cuba might try to flee from the Communist state.
U.S. officials are urging both groups to remain where they are, and not attempt any trips across the 150 kilometers of open water between the Caribbean island and the southern United States.
A State Department spokesman Sean McCormack says the United States stands ready to help the Cuban people if they show a desire to transition to democracy.
In another development today, North Korea says its leader, Kim Jong Il, sent a message wishing Mr. Castro a speedy recovery so he can continue his leadership of Cuba, which began in 1959. North Korea and Cuba are two of the world's few remaining Communist states.
Information for this report is provided by AP, AFP and Reuters.
