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Mr. Chavez has run Venezuela for more than 14 years as a virtual one-man show, gradually placing all state institutions under his personal control. But the former army paratroop commander, who rose to fame by launching a failed 1992 coup, never groomed a successor with his same kind of force of personality.

Mr. Chavez was last re-elected on Oct. 7, and his challenger, youthful Miranda state Gov. Henrique Capriles, is expected to again be the opposition’s candidate in any new election.

One of Mr. Chavez’s three daughters, Maria Gabriela, expressed thanks to well-wishers via her Twitter account.

“We will prevail!” she wrote, echoing a favorite phrase of her father. “With God always.”

Mr. Maduro said last week that the president had begun receiving chemotherapy around the end of January.

Doctors have said such therapy is not necessarily to beat Chavez’s cancer into remission but could have been palliative, to extend Mr. Chavez’s life and ease his suffering.

Dr. Carlos Castro, scientific director of the Colombian League Against Cancer, said “it’s difficult to predict” when Mr. Chavez might die, but he believes “it’s a matter of days.”

Dr. Castro said Mr. Chavez could face further respiratory complications if he receives more intense chemotherapy treatment.

If the president’s medical team “gives him strong chemotherapy again, then it would not be surprising if some infections reappear,” Dr. Castro said in a telephone interview.

While in Cuba, MR. Chavez suffered a severe respiratory infection in late December that nearly killed him, MR. Maduro said last week. A tracheal tube was inserted then, and government officials have said his breathing remained labored.

Libardo Rodriguez, a 60-year-old orange juice vendor, said he was very worried after Monday evening’s announcement and the government should provide more information about Mr. Chavez.

“We are worried because he does not appear. The truth is that I don’t know what’s happening,” said Mr. Rodriguez, who identified himself as a Chavez supporter.

“There are many rumors, and nobody knows who to believe,” he said. “We hope he’s alive.”

• Associated Press writer Vivian Sequera in Bogota, Colombia, contributed to this article.