MUNICIPIO INDEPENDENCIA, Venezuela - President Hugo Chavez said Thursday his close friend and ally Fidel Castro is in a "great battle for life," but he also expressed optimism about the 79-year-old Cuban leader's recovery.

"From here, let's pray to God for Fidel and his recovery, and he's fighting a great battle," Chavez said in a televised speech from the eastern state of Anzoategui.

His statement was the most serious yet from a close Castro ally in describing the 79-year-old Cuban leader's condition. But Chavez also predicted, with a laugh, that Castro would defy the U.S. government's plans for a transition in Cuba and emerge from a hospital where he is being treated to reassume the presidency.

Chavez said he had received a message from Castro on Wednesday "that filled me with more optimism, with more faith."

"Among other things Fidel told me ... 'I keep saying Chavez, God help Chavez and his friends,'" Chavez said.

"I wrote to him in my own handwriting last night, in the early morning, to send it with the messenger who was returning immediately: 'You are fighting a great battle every day, all these nights,'" Chavez said.

Castro said July 31 he was stepping aside temporarily, granting his powers to his brother Raul as head of the government and the Communist Party so he could recover from intestinal surgery.

Neither brother has been seen in public since then. Details of Castro's condition, his ailment and the surgical procedure he underwent are being treated as a "state secret."

Chavez said that in his letter to Castro, "I told him, 'Here we're with you every second, every minute, every hour, every day, every night of that great battle for life that you are fighting from your heart, from your soul.'"

"It's a battle, and I know, Fidel, that we're going to win it, too. We are prevailing and we will prevail," Chavez said.

Chavez spoke near the end of his speech as if Castro were listening.

"Get better, Fidel. It's an order. I never give you an order. Now I'm giving you one," Chavez said. "Get better, an order. I know you're disciplined."

The Venezuelan president described Castro's health troubles as an "ambush," coming so soon after Castro gave a three-hour speech in Cordoba, Argentina, and visited the childhood home of fellow revolutionary Ernesto "Che" Guevara.

Chavez said he and Castro hugged at the door of the plane, promised to see each other again soon, "and then suddenly the surprise, but that's life."

"It's life, biology," Chavez said. "And it's also 80 years that you've lived, Fidel Castro, and what an 80 years Fidel has lived."

The Venezuelan leader has said he will dedicate his weekly TV and radio program, "Hello President," this Sunday to Castro in honor of his 80th birthday.