Before boarding the presidential plane at Caracas' international airport, Chavez said: "It's not time to die. It's time to live."
He stood by one of his daughters, Rosa, on a red carpet leading to the plane as a military band played the national anthem and soldiers stood at attention.
"I'm saying goodbye for some days," Chavez said, "but in a deeper sense I'm not saying goodbye. I'm going to be in the same homeland, in the great homeland. And I'll be attentive every day, every hour, every minute to internal events and I'll be in permanent contact."
He then climbed the stairs of the plane holding the hand of his daughter, who left with him. He has not said how long he expects to stay in Havana.
The 56-year-old leftist leader spent much of Saturday discussing his health and other issues, including during a televised speech at the presidential palace.
"Tomorrow I begin chemotherapy treatment," he said, "and we're going to give it everything we've got." He said the treatment would ensure cancer cells have not reappeared since he underwent surgery last month to remove a tumor.
After thorough medical checks, "no malignant cell has been detected in any other part of my body," Chavez said to the applause of aides and allies. He added there is always a risk cancer cells might reappear, "and therefore there's a need to attack hard through chemotherapy."
Chavez also made contingencies for his absence by delegating some of his duties to Vice President Elias Jaua and Planning and Finance Minister Jorge Giordani.
Chavez previously had refused opposition demands that he temporarily cede power to Jaua while undergoing chemotherapy. But shortly after a legislative vote approving his trip Saturday morning, Chavez said at a televised Cabinet meeting that he would hand off some administrative responsibilities.
"Chavez is now going to be: Chavez delegating, much more than before," he said.
Chavez said his decision to entrust some of duties to aides was a result of "deep reflection" in recent days.
He said his vice president would oversee budget transfers to government ministries, presidential commissions, any approved expropriations of businesses and other budget-related responsibilities. Giordani would deal with matters including budget shortfalls and certain tax exemptions.
Chavez denied he was in any way ceding his functions as president.
He said that if his physical capacities were diminished in the future, "I would be the first in doing what the constitution says" in delegating his position to the vice president.
In a speech to party leaders and aides Saturday afternoon, he called for them to defeat any internal divisions, describing such conflicts as "cancerous tumors within the political body."
"Unity, unity, unity," Chavez said.
He repeated that message later as he addressed troops and supporters on the steps of the presidential palace. He announced new appointments for five generals including the chief of his presidential guard, saying the moves were to "continue strengthening the unity of the Armed Force."
"Military unity, civilian unity. ... National unity. That's one of the greatest ways you can help me now," Chavez said. "I will return, and I'll return better than I'm going away."
Some analysts have said there appear to be divisions between some military and civilian allies of the president.
Chavez, who has held dominant power during more than 12 years in office, has said he's confident he will rebound but has also acknowledged a long road to recovery remains.
Chavez underwent surgery in Cuba on June 20 to remove a cancerous tumor, which he has said was the size of a baseball. He hasn't said what type of cancer he was diagnosed with nor specified where exactly it was located, saying only that it was in his pelvic region.
The National Assembly had to first approve his departure, and the issue raised a passionate debate in which opposition politicians said they supported the president's right to receive treatment but disputed his plan to remain in charge while in Havana. Opposition lawmakers also demanded more information about his illness.
As the debate was under way, Chavez appeared on television and interjected himself into the debate. He appeared on a split screen with the lawmakers' listening, dismissing his opponents' arguments as "bordering on ridiculousness."
Opposition lawmakers, who hold a minority of seats in the National Assembly, said they believed that Chavez's request constituted a "temporary absence" and that the president owed the country a more detailed explanation of how serious his illness is.
"Let him go to Cuba," opposition lawmaker Alfonso Marquina said during the debate. "But we also demand compliance with the constitution ... so that he doesn't continue governing from Havana."
Under Venezuela's constitution, the vice president may take the president's place during temporary absences of up to 90 days, which the National Assembly may extend for 90 days more, for a total of about six months.
Pro-Chavez lawmaker Cilia Flores said the National Assembly was simply granting Chavez permission to be away for more than five days and that he would remain in charge.
Chavez spent much of June in Cuba undergoing surgeries to remove an abscess and the tumor. He made a surprise return from Havana on July 4 and has since rallied supporters, addressed troops and generally sought to reassure Venezuelans that he is in control in spite of his illness.
Chavez acknowledged on Wednesday for the first time that he expected to undergo chemotherapy or radiation treatment.
Since coming to power in 1999, Chavez has launched sweeping socialist-inspired changes, built alliances with Latin America's leftist leaders and frequently clashed with the U.S. government, although Venezuela still relies on income from oil sales to the United States, which remains its top client.
The leftist leader is up for re-election in late 2012, and he told members of his party Saturday: "We have to obtain a great victory."
"Undoubtedly, the opposition will see his return to Cuba as a further sign of frailty," said Miguel Tinker Salas, a Latin American studies professor at Pomona College in Claremont, California. "Among his supporters, it might have the opposite outcome, serving to rally support and close ranks."
With a rough regimen of medical procedures awaiting him, Chavez repeatedly insisted he would survive and return.
During a send-off at the presidential palace, newly appointed Youth Minister Maria Pilar Rodriguez played a guitar and sang the song "Solo El Amor," or "Only Love" by Cuban folk singer Silvio Rodriguez.
"Only love begets wonder," she sang. "Only love brings light to that which is dead."
Before he stepped aboard the jet, Chavez said: "Toward life always, my beloved Venezuela."
