The Pope asked not to return to hospital for treatment
No word has been heard for more than 12 hours on his condition, but a briefing has been scheduled for 0930GMT.
At the last update, his breathing was reportedly shallow, his blood pressure low and there were problems with both his heart and kidneys.
Thousands of people held an evening vigil in St Peter's Square.
Up to 70,000 attended past midnight, but numbers had dwindled to just a few hundred by the early hours.
"Stay with us. Don't leave us," the crowds chanted, many crying uncontrollably.
This evening or this night, Christ opens the door to the Pope
Angelo Comastri
Vicar general of Vatican City
Vatican statements What is Anointing the Sick?
The Vatican made three statements on Friday, seemingly to prepare Catholics to expect the worst.
Unusually, it kept its press centre open all Friday night, but closed it at 0600 (0400 GMT) on Saturday, saying it would reopen at 0900.
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.
"I invite all Romans and all Italians to intensify their prayers for him," he said.
"We want to be close to him in this hour through the same loving closeness with which John Paul II has accompanied us for nearly 27 years."
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."
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.
Krakow people pay tribute to the Pope they called their own
In pictures
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.
Soon he will carry his cross into eternity and we will have to study hard the footprints he has left
David Power
Tipperary, Ireland
Pope's illness: Your views
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."
US President George W Bush and his wife Laura had joined those around the world who were praying for him, White House spokesman Scott McClellan said.
Stock image of Pope John Paul II in the Vatican
'''John Paul II (Karol Józef Wojtyła) 1920-2005'''

After months of failing health, ''His Holiness'' Pope John Paul II passed away today, April 2, at 9:37 p.m. local time (19:37 UTC). He was 84 years old.  An email message to journalists from the spokesman Joaquin Navarro-Valls from The Vatican confirmed the death at 20:00 UTC.

The Pope had a  earlier this year to help him breathe, and then lost his ability to speak last week. On Thursday March 31 he developed a urinary tract infection that led to septic shock and cardiovascular collapse.  A nasogastric feeding tube was inserted to help him regain strength.  The Pope elected not to return to Rome's Gemelli hospital, but instead to stay in his own apartments where he was tended to by his personal doctors and Vatican medical staff.

The same night, he was administered the '''' also known as the last rites. On Friday April 1, the Vatican said he was having difficulties with both his heart and his kidneys.

 news agency Agenzia Giornalistica Italia reports the pope's final message was "I am happy, you be happy, too. Let's pray together with joy. I entrust everything to the Virgin Mary with joy".

A description of the last moment of the holy father from father Jarek Cielecki, director of the Vatican service news and of the Italian TV catolic Tele Padre Pio, tells us the last moment of life of this great man. "The Holy father died looking at the window, gathered in prayer. As such he was conscious. Just before dying, the Pope raised his right hand in a sign of blessing, as if he was aware of all the people gathered in prayer. Then, as soon as the prayer ended, the Pope did a huge effort, said the word 'Amen' and died."

The Vatican has announced that the funeral will take place on Wednesday, April 6 and that the Pope's body will lie in state from Monday, April 4.