Get-well wishes poured in from leftists across the hemisphere.

Former Nicaraguan President and Sandinista revolution leader Daniel Ortega arrived in Havana late Saturday. "I am sure that we will soon have Fidel resuming his functions and leading his people," Ortega said.

Colombia's largest rebel group also expressed its solidarity with the Cuban leader. "We hope you'll recover in the shortest time possible," the Revolutionary Armed Forces of Colombia said in a statement.

Secretary of State Condoleezza Rice said Sunday the United States wanted to help Cubans prepare for democracy but was not contemplating an invasion of the island in the wake of Castro's illness.

"The notion that somehow the United States is going to invade Cuba, because there are troubles in Cuba, is simply far-fetched," Rice told NBC News. "The United States wants to be a partner and a friend to the Cuban people as they move through this period of difficulty and as they move ahead. But what Cuba should not have is the replacement of one dictator by another."

Cuban authorities have beefed up security by mobilizing citizen defense militias, increasing street patrols and ordering decommissioned military officers to check in daily.

Roman Catholic Cardinal Jaime Ortega, the island's top churchman, called on parishioners Sunday to pray for Castro's health, peace on the island and fraternity among all Cubans, both here and abroad.

"We pray for the fatherland, for Cuba, and those who are leading it," Ortega told reporters after Sunday Mass at the cathedral in Old Havana.

Outside another church, a group of political prisoners' wives known as the Ladies in White held their weekly silent march without interruption by authorities.