Castro said the news, although not unexpected, had been a hard blow.

"On the 5th of March, in the afternoon hours, died the best friend the Cuban people had in their history," Castro wrote in a column published in Communist Party newspaper Granma.

"We have the honor of having shared with the Bolivarian leader the same ideals of social justice and of support for the exploited," said the 86-year-old Castro who led Cuba's 1959 revolution, ruled the country for 49 years and still plays a behind-the-scenes role.

"The poor are the poor in any part of the world," he said.

Castro said he had received a phone call via satellite notifying him of what he called "the bitter news."

"The significance of the phrase used was unmistakable. Although we knew the critical state of his health, the news hit us hard," wrote Castro, who resigned as Cuba's president five years ago because of his own health problems.

"I remembered the times he joked with me saying that when both of us finished our revolutionary work, he would invite me to spend time by the Arauca River in Venezuelan territory, which reminded him of the rest he never had," Castro said.

Castro closed his column by paraphrasing a famous quote from another late friend and revolutionary, Ernesto "Che" Guevara.

"Until victory always, unforgettable friend," Castro wrote of Chavez.