VATICAN CITY (AP) — 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 failed yesterday afternoon.

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

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

The 84-year-old pontiff’s health declined sharply after he developed a high fever yesterday brought on by the infection. He wished to remain at the Vatican, Navarro-Valls said.

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

St. Peter’s Square was quiet this morning with a few tourists and pilgrims stopping to look up at the pontiff’s third floor window. As always Swiss guards in the colorful uniforms stood by at the open bronze door, which by tradition is closed upon the death of a pontiff.

Earlier hundreds of people had gathered, 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.

Formerly called the last rites, the sacrament is often misunderstood as signaling imminent death. It is performed, however, not only for patients at the point of death, but also 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 on 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 had attended Mass yesterday 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.

Navarro-Valls told The Associated Press 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. Vatican radio later described the pope as stable and responding to the antibiotics.

The Vatican medical staff appeared confident it could handle the crisis with the sophisticated medical equipment installed for the pontiff.

After his heart failed, the pope was provided with “all the appropriate therapeutic provisions and cardio-respiratory assistance,” today’s statement said.

It said that the pope was being helped by his personal doctor, two intensive care doctors, a cardiologist, an ear, nose and throat specialist and two nurses.

A heart failure occurs when the heart no longer has the strength to pump blood through the body, and is a sign that the body’s cardiac system is failing.

Dr. Paolo Nardini, a Rome physician who is not part of the pope’s team, said “a heart attack, which is very serious, affects only the heart, while heart failure signals a breakdown of the entire system, basically uncurable.”

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.

John Paul’s 26-year papacy has been marked by its call to value the aged and to respect the sick, subjects the pope has turned to as he battles Parkinson’s disease and crippling knee and hip ailments.

It is not clear who would be empowered to make medical decisions for an unconscious pope. The Vatican has officially declined to comment whether John Paul has left written instructions.

A urinary infection can produce fever and a drop in blood pressure as reported in the pope, said Dr. Marc Siegel, a specialist in internal medicine at the New York University Medical Center.