Washington's "only purpose with these bases is the ability to put US troops in South America in a matter of hours," Castro said in an article published on the official government website cubadebate.cu.
The United States insists the facilities, spread across the territory of its main regional ally, are aimed at fighting drug gangs and left-wing rebels in Colombia.
Castro said America's real objective was to "eliminate the revolutionary process" begun by Venezuela's firebrand leader Hugo Chavez, a key Cuban ally, and to "gain control of the oil and other natural resources in Venezuela.
"The delivery of land to establish seven US military bases in Colombia directly threatens the sovereignty and integrity of the peoples of South and Central America and the great Latin American fatherland our forefathers dreamed of," he wrote.
Chavez, who is fond of fierce anti-US rhetoric, has said the bases could be a prelude to an invasion of his oil-rich nation and has spoken of "winds of war" blowing across the region because of the deal.
The lead-up to Friday's regional summit in Argentina's ski resort town of Bariloche has been marred by sharp exchanges between Colombia and the main critic of the plan, Venezuela.
Most of South America -- including Venezuela's allies Ecuador and Bolivia, as well as more moderate nations Brazil, Chile and Argentina -- fear they could be used for military incursions into neighboring countries. Only Peru has supported Colombia.
Castro, 83, transferred power of communist-led Cuba to his younger brother Raul three years ago because of illness.
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