Venezuelan President Hugo Chavez faces a "complex and difficult" recovery from cancer, his vice-president has said.
Nicolas Maduro, Mr Chavez's preferred successor, said the president's latest surgery in Cuba had been "complex, difficult, delicate".
He urged Venezuelans to pray for the 58-year-old president, and to face the "hard days" ahead calmly.
Mr Chavez is in stable condition after his fourth operation, Information Minister Ernesto Villegas said.
Mr Maduro was speaking alongside National Assembly President Diosdado Cabello and Oil Minister Rafael Ramirez, who had both joined Mr Chavez on his trip to Cuba. The president's children and grandchildren were also with him in Havana.
A solemn Mr Maduro, his voice cracking, said: "It was a complex, difficult, delicate operation," which meant that "the post-operative process is also going to be a complex and hard process".
Before Mr Maduro spoke, state television showed footage of Venezuelans praying for the president at a Mass.
Mr Chavez remains in Cuba and there has been no indication when he will return.
Ecuadoran President Rafael Correa, who had also flown to Havana to be with Mr Chavez, said it had been a "very delicate operation", adding: "He gives us all strength."
Mr Maduro, 50, had earlier described the surgery to remove cancerous tissue, which lasted more than six hours, as a "complete success".
Mr Chavez had returned from cancer-related treatment in Cuba last Friday but left again for surgery early on Monday.
The illness was first detected during an initial surgery for a pelvic abscess in June 2011.
During the weekend, Mr Chavez himself acknowledged the seriousness of the situation.
"There are risks. Who can deny it?" he said in a televised speech.
Mr Chavez added that if his health failed and new elections had to be held, people should vote for Mr Maduro, a former bus driver and one of the president's closest advisers.
"He is a complete revolutionary, a man of great experience despite his youth, with great dedication and capacity for work," Mr Chavez said.
Mr Chavez, who was re-elected in October for a fourth term as president, is due to begin his new six-year term in office on 10 January.
The constitution states that should the president leave office in the first four years of his term, an election must be held within 30 days.