"The clinical conditions of the Holy Father remain very serious. In late morning, the high fever developed. When addressed by members of his household, he responds correctly," the Holy See said in a statement.
The 84-year-old pontiff was reported to have had a fever on Thursday night which the Vatican blamed on a urinary tract infection that later led to heart and kidney failure. The Holy See did not say whether the fever had subsided at any time since or whether Saturday morning's fever was a new bout.
At about the time the communique was issued, a light went on in the pope's third-floor apartment overlooking St. Peter's Square, where an estimated 40,000 people were keeping vigil.
Papal spokesman Joaquin Navarro-Valls said earlier in the day that John Paul was not in a coma and opened his eyes when spoken to. But he added: "Since dawn this morning, there have been first signs that consciousness is being affected."
"Sometimes it seems as if he were resting with his eyes closed, but when you speak to him, he opens his eyes," Navarro-Valls said.
He said aides had told the pope that thousands of young people were in St. Peter's Square on Friday evening. Navarro-Valls said the pope appeared to be referring to them when he seemed to say: "'I have looked for you. Now you have come to me. And I thank you."'
CBS News Correspondent Allen Pizzey reports that few people expect the pope to recover from his recent spiral of failing health, which is causing his organs to fail.
"His dying is going in such a serene way," Pizzey said, adding that the pope's words, as described by Navarro-Valls suggest a message to youth that "life has extreme value."
"He's going: He knows it," Pizzey said. "Everyone knows it."
Vatican cardinal Achille Silvestrini visited John Paul Saturday morning, accompanied by another cardinal, Jean-Louis Tauran.
"I found him relaxed, placid, serene. He was in his bed. He was breathing without labor. He looked like he lost weight," Silvestrini said.
He said that when he and Tauran came into the room, the pope seemed to recognize them.
"The pope showed with a vibration of his face that he understood, indicating with a movement of his eyes. He showed he was reacting," he added.
For a second day, the Vatican announced a series of papal appointments including a Spanish bishop, an official of the Armenian Catholic Church and ambassadors to El Salvador and Panama.
One of the pope's closest aides, Cardinal Joseph Ratzinger, was quoted Saturday as saying that when he saw the pontiff on Friday morning, John Paul was "aware that he is passing to the Lord."
The pope "gave me the final farewell," the news agency of the Italian bishops conference quoted the German cardinal as saying Friday night.
Tourists and pilgrims streamed anew into St. Peter's Square on Saturday, and around the world, priests prepared Roman Catholics for the pope's death. Many expressed hope that his final hours would be peaceful.
"Now he prepares to meet the Lord," Cardinal Francis George said at a Mass in Chicago on Friday. "As the portals of death open for him, as they will for each of us ... we must accompany him with our own prayers."
On Saturday, President Bush called the pope "a faithful servant of God and a champion of human dignity and freedom" and said millions of Americans were praying for him.
A workman at the Vatican, declining to give his name, told The Associated Press that crews were taking down the canopy on the steps of St. Peter's Basilica, which had covered an altar during Easter Sunday Mass. They said they had orders to clear the space for when the pope's coffin eventually is carried into the square.