Pope John Paul II's poor health has become even worse, the Vatican said.
His breathing is shallow, his blood pressure is low and he is having difficulties with both his heart and his kidneys, a spokesman said.
Millions of Catholics around the world have been praying for the Pope, and a special Mass was held for him in Rome where crowds are maintaining a vigil.
The Vatican made three statements in one day and seems to be preparing Catholics to expect the worst.
Cardinals - who will elect a new pope after John Paul dies - are arriving in Rome from all over the world, the Italian media have reported.
The Pope's vicar general of Rome said the pontiff was already "at one" with Jesus Christ.
He "already sees and touches the Lord," Cardinal Camillo Ruini told a packed Mass in Rome's St John Lateran cathedral.
Another senior clergyman, Angelo Comastri, told about 30,000 people gathered in St Peter's Square: "This evening or this night, Christ opens the door to the Pope."
He later denied reports in the Italian media that the Pope had died.
Sacrament given
Vatican spokesman Joaquin Navarro-Valls said on Friday evening that the Pope's overall condition had been "notably compromised".
But he added that the Pope was visibly participating in prayers around his sickbed.
The Pope's condition deteriorated on Thursday after he developed a urinary tract infection that later brought on "septic shock and a cardio-circulatory collapse".
He was given the Catholic sacrament for the sick and dying - called the Anointing of the Sick.
But the Pope decided not to return to Rome's Gemelli hospital.
He was being treated in his apartment by a team of four top consultants and his private doctor Renato Buzzonetti.
Homeland homage
Catholics around the world have been praying for the Pope.
In the Pope's homeland, Poland, churches have been packed with worshippers throughout the day.
The Archbishop of Krakow, Franciszek Macharski - a long-time friend of the Pope - told his audience: "Do not feel shame at showing your emotion and at shedding tears."
More than 20,000 Brazilians gathered for a Mass in Sao Paulo, in what the city's archbishop described as an expression of love.
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