Vatican City ? Pope John Paul II suffered heart failure during treatment for a urinary tract infection and was in “very serious” condition today, the Vatican said.

Vatican spokesman Joaquin Navarro-Valls said in a statement that the pope, who was being treated at the Vatican, was given cardio-respiratory assistance after his heart stopped Thursday afternoon.

“This morning the condition of the Holy Father is very serious,” the statement said.

However, it said the pope had participated in a 6 a.m. Mass today and was “conscious, lucid, and serene.”

The pontiff’s health declined sharply after he developed a high fever Thursday brought on by the infection. His wish to remain at the Vatican was respected, Navarro-Valls said.

The pontiff was attended to by the Vatican medical team, and provided with “all the appropriate therapeutic provisions and cardio-respiratory assistance,” the statement said.

The statement confirmed previous reports that the pope had received the sacrament for the sick and dying Thursday evening.

At the edge of St. Peter’s Square, hundreds of people gathered early today, concerned about the fragile pope. A few knelt on the cobblestones to pray, others wrapped blankets around themselves as they kept vigil through the night.

“There’s nothing we can do but pray. We’re all upset,” said Agriculture Minister Giovanni Alemanno, who was in the crowd.

As Pope John Paul II was treated for a high fever late Thursday, a nun prays in St. Peter's Square at the Vatican. The pope suffered heart failure while being treated for a urinary tract infection, and was being treated at the Vatican with antibiotics, the pope's spokesman said. The development came one day after the 84-year-old pontiff began receiving nutrition through a feeding tube.

Formerly called the last rites, the sacrament is often misunderstood as signaling imminent death. It is performed, however, for patients at the point of death and for those who are very sick — and it may be repeated.

The Rome daily La Repubblica reported today that the sacrament was administered by John Paul’s closest aide, Polish Archbishop Stanislaw Dziwisz, who serves as his private secretary. Dziwisz had given the pontiff the same sacrament Feb. 24 just before the pope underwent a tracheotomy to insert a tube in his throat at Gemelli Polyclinic, the newspaper said.

According to its account, John Paul attended Mass on Thursday morning in his private chapel, then did paperwork from an armchair. Abruptly, at 6:45 p.m., John Paul turned ghostly pale and his blood pressure plummeted, the newspaper said.

After antibiotics were administered, the Italian news agency Apcom reported without citing any sources, John Paul’s condition was “stable.” ANSA, another Italian news agency, said the pope “seems to showing a first positive reaction” to antibiotic therapy.

At the Gemelli hospital, an emergency room chief said there were no plans to admit John Paul “at the moment,” ANSA reported.

While the pope’s condition deteriorated suddenly, the Vatican medical staff appeared confident it could handle the crisis with the sophisticated medical equipment installed for the pontiff.

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As a crowd of people gathered to keep vigil near the pope, police kept them off St. Peter’s Square, forcing them to congregate at the edges of the giant plaza.

“I was in the car and I heard on the radio about the grave condition of the pope. I immediately thought I would come to St. Peter’s,” said Antonio Ceresa, a Roman.

As news of the pope’s latest health crisis swept the world, the pontiff’s spokesman, Navarro-Valls, said by telephone that “the Holy Father today was struck by a high fever caused by a confirmed infection of the urinary tract.”

The pontiff was started on “an appropriate” course of antibiotics, Navarro-Valls said. “The medical situation is being strictly controlled by the Vatican medical team that is taking care of him.”

Hospitalized twice last month following two breathing crises and with a tube placed in his throat to help him breathe, John Paul has become a picture of suffering. When he appeared at his apartment window Wednesday to bless pilgrims in St. Peter’s Square, he managed to utter only a rasp.

Later that day, the Vatican announced he had been fitted with a feeding tube in his nose to help boost his nutritional intake.