Cuba's armed forces have begun their biggest military exercise for nearly 20 years, involving hundreds of thousands of troops and civilians.
Cuba's defence minister is Raul Castro, right - Fidel's brother
The exercise, called Bastion 2004, is a response to "the continued aggression and threats" of the US, Cuba says.
The manoeuvres are being led by Defence Minister Raul Castro - brother to President Fidel Castro and second in the Cuban hierarchy.
The US said the drill was an attempt to "distract" Cubans from daily hardships.
The exercise will involve exercises with Cuban air force MiG-29s, anti-aircraft units and elite troops.
Two days of civil defence exercises at the end of the week-long exercise will include a simulated aerial assault.
Officials say about 100,000 soldiers, some 400,000 reservists, and "millions" of civilians will take part.
Many Cuban civilians already belong to the thousands of neighbourhood Committees for the Defence of the Revolution, whose duties include maintaining readiness against attack.
"The determination of the US administration to destroy the [Cuban] revolution however they can, including militarily, determines the necessity of conducting these exercises," said Gen Leonardo Andollo, deputy chief of the armed forces, according to the AFP news agency.
Last week, the defence minister said the exercises would ensure "the enemy does not underestimate" the Cuban people.
The US sponsored a failed attempt by Cuban exiles to invade Cuba at the Bay of Pigs in 1961 but US officials are scornful at the idea that another invasion could be mounted.
"We don't think there is any justification, or any
particular foundation for this kind of charge," said Richard Boucher, spokesman for the US state department.
"The United States has repeatedly called for a peaceful transition to democracy in Cuba. We think that's what the Cuban people deserve, and we think they deserve it in a peaceful fashion," he was quoted as saying by AFP.
These "exercises are just, I would say, one or more of the many things that the Cuban government does to try to distract people from the problems that they face in their daily lives".
The US and Cuba have exchanged a series of verbal blows over the past year, with the US tightening its tough sanctions on the island and Cuba retaliating by imposing a tax on the use of US dollars.
Venezuelan President Hugo Chavez was expected to arrive in Havana on a two-day visit on Monday, reports said - his second visit in just over one month. There were few details on his itinerary.