Pope Benedict XVI urged tens of thousands of Cubans gathered for an open-air mass to "build a renewed and open society," at the start of his visit to the Communist-ruled country.

The pontiff -- making the first papal visit to Cuba in 14 years -- is seeking to bolster close church-state ties there while also urging authorities in the Americas' only one-party Communist state to embrace change.

Venezuelan President Hugo Chavez flew back to Cuba late Saturday for more radiation treatment after his latest cancer surgery.

Surrounded by military guards of honor, the Venezuelan leader boarded a Havana-bound plane at Maiquetia International Airport at about 9:15 pm local time (0145 GMT Sunday).

President Hugo Chavez returned home to Venezuela looking haggard but expressing optimism he will survive after undergoing cancer surgery in Cuba.

Chavez said he has been praying and is confident he will triumph in his re-election bid this year. He smiled and waved as he stepped off the plane Friday night, but his expression turned sober as he referred to his unfinished struggle with cancer.

Cuba's top biotech teams have successfully tested a new AIDS vaccine on mice, and are ready to soon begin human testing, a leading researcher told a biotechnology conference in Havana on Monday.

"The new AIDS trial vaccine already was tested successfully (on mice) and now we are preparing a very small, tightly controlled phase one clinical trial" with HIV-positive patients who are not in the advanced stages of disease, researcher Enrique Iglesias said.

Venezuelan President Hugo Chavez assured supporters in a flurry of Twitter messages from Cuba Saturday that he was recovering well from surgery to remove a potentially cancerous lesion.

He underwent surgery on the same area of his pelvis where a cancerous tumor was removed last June.

Venezuelan President Hugo Chavez flew to Havana Friday for treatment of what he says is likely a malignant lesion in the same part of his body where Cuban doctors removed a cancerous tumor in 2011.

The 57 year-old Chavez, who is seeking re-election in October, vowed to defeat "this new difficulty" just ahead of boarding his flight to Havana, state media reported.

President Hugo Chavez has referred to the opposition candidate for Venezuela's highest office as a "low-life... pig" in a sharp attack at the start of this year's election season.

"My mission... (will be) to take off the mask, you low-life, because no matter how much you disguise it, low-life, you have a pig's tail, a pig's ears, and you snort like a pig," Chavez said Thursday in a speech broadcast on state radio and television, referring to opposition candidate Henrique Capriles.

Cuban leader Fidel Castro is publishing two volumes of his autobiographical memoir entitled "Time Guerrilla," the Cuban news media reported Saturday.

The memoirs trace his life from infancy until 1958, when he succeeded in leading a revolution that turned Cuba into a communist country aligned with the Soviet Union.

After his brother led Cuba for almost five decades, President Raul Castro announced at a Cuban Communist Party event Sunday that political terms would be limited to 10 years -- even his own.

"We can begin implementing this slowly without waiting for a constitutional reform" that is legally necessary, the president, 80, told the special conference trying to re-energize a party that has run the island uncontested for five decades, but is under mounting pressure to boost the pace of change.

Cuban dissident Wilmar Villar was buried in his hometown in eastern Cuba on Friday, rights activists told AFP, as Havana denied the inmate was a political prisoner who died after a 50-day hunger strike.

Villar, 31, is the second dissident to die here after a hunger strike in less than two years, and his passing drew condemnation from national and international human rights groups, as well as foreign governments.