Pope John Paul II remains gravely ill after suffering heart problems overnight, the Vatican has said.
The Pope asked not to return to hospital for treatment
But he is said to be conscious and was able to celebrate Mass on Friday morning and approve several church appointments later in the day.
The Pope was given the Catholic sacrament for the sick and dying after his health deteriorated on Thursday.
Millions of Catholics around the world are praying for the Pope. A Mass is due to be held on Friday evening in Rome.
The decision to give the Pope the sacrament - called the Anointing of the Sick - is a sign of how seriously ill he is, say correspondents.
The Vatican announced that the Pope's health had worsened on Thursday when he developed a urinary tract infection that later brought on "septic shock and a cardio-circulatory collapse".
He was "immediately informed about the gravity of his condition", papal spokesman Joaquin Navarro-Valls told reporters at the Vatican on Friday.
But the Pope decided not to return to Rome's Gemelli hospital - and is being treated in his apartment by a team of four top consultants and his private doctor Renato Buzzonetti.
On Friday morning, he was "still lucid, fully conscious and extraordinarily serene", the spokesman said.
The Pope was able to make a sign of the Cross on Friday morning as aides read the scriptures to him, he added.
Pilgrims gather in St Peter's Square to pray for the Pope
Later on Friday, the Vatican announced that the Pope had approved the appointment of 17 new bishops and archbishops and accepted the resignation of six others.
Millions of Catholics across the world have been gathering in churches and in the open air to pray for the Pope.
In Rome, the Pope's vicar Cardinal Camillo Ruini is due to preside over a mass at St John Lateran Basilica, starting at 1900 local time (1700 GMT).
Police have imposed traffic restrictions around the Vatican and authorities are making plans to deal with a huge influx of pilgrims anxious to be present in the event of an announcement of the Pope's death.
Cardinals - who will have the duty of electing a new pope - are beginning to arrive in Rome from all over the world, Italian media have reported.
'Expression of love'
In the Pope's homeland, Poland, churches have been packed with worshippers.
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.
Senior clergy are openly expressing pessimism about the possibility of the Pope ever resuming the guidance of his one billion-strong Church.
He has Parkinson's disease, an incurable neurological condition from which he has been suffering for nearly a decade.
The Pope appeared briefly at the window of his Vatican apartment on Easter Sunday to bless the faithful, but was not able to speak.
He tried again to speak a few days later - a sign of his extraordinary strong will, our correspondent says.
So far this year, the Pope has had two spells in hospital where he received treatment for breathing problems and underwent an operation to insert a breathing tube.