A police mobilisation in Havana was accompanied by a tightening of security at exit points, and a mobilisation of Communist Party activists and rapid response brigades.
"Today there exists an opportunity for courageous men and women who want Cuba to follow a different path," said Jorge Mas Santos president of the Cuban American National Foundation. "They should take advantage of this opportunity."
Mr Mas Santos said he could envisage a military revolt or civilian uprising that "will put Cuba on the path toward democracy."
Cubans have been forced to ponder the fate of their leader of nearly five decades after he underwent gastrointestinal surgery.
A top Cuban official said that Fidel Castro remained "very alert" following his operation, and continued to closely follow domestic and international affairs.
But neither he nor his brother Raul, whom he named as his temporary replacement Monday, appeared in public, stoking concerns over the country's future.
Castro conscious
National Assembly Speaker Ricardo Alarcon told US-based Democracy Now radio that he had spoken on Monday and Tuesday with Fidel Castro.
"He is perfectly conscious, very good speech as always. We talked for over a half an hour on many things going on in the world, the impact that the announcement has had," Mr Alarcon said.
He said that they had also spoken about the situation in Lebanon and issues referring to the United States.
"Of course, he is forced to have a period of rest. He underwent complicated surgery. He's in, I would say, a normal period of recovery after an important surgery," said Mr Alarcon.
"But he is very alive and very alert as always, very interested in what's going on around him and around the world," he said.
Castro's absence from public view has fuelled speculation that he might be gravely ill, if not already dead.
Since assuming his brother's positions as first secretary of the Central Committee of the Communist Party, commander in chief and president of the government, Raul Castro, 75, has also not addressed the Cuban people or appeared in public.
No photo-ops
Mr Alarcon has assured the public that Fidel Castro's last moment was "far away," and that even after a delicate operation "he keeps track of the smallest detail and takes steps to confront any enemy aggression."
Asked about what is wrong with Castro medically, Mr Alarcon told the US radio simply: "He works a lot."
"Apart from being substituted provisionally, he also had to designate certain comrades for specific programs on health care, education and energy and so on -- programs that he was conducting personally."
"I would say that he is doing fairly well. I don't want to diminish the seriousness of the situation. ... He needs a lot of care and attention" after surgery, Mr Alarcon said.
"That is why he cannot be at the same time holding certain positions. For him, that is not a photo op, it is a matter of hours and hours dedicated to health care programs, education programs."
Mr Alarcon added: "He is resting. He has to do that. ... He is not a young fellow. He is 79 years old."
Many analysts said the communist government appeared to be rehearsing its transition strategy, possibly to build public confidence in Raul Castro. His personality is far less known than his charismatic brother.