Pope John Paul II has suffered serious heart problems and is in a grave condition, the Vatican has announced.
The Pope has received the last rites
The urgent communique came hours after the Pope's health worsened with a high fever caused by an infection.
"Following a urinary tract infection, septic shock and a cardio-circulatory collapse occurred," Vatican spokesman Joaquin Navarro-Valls said.
On Thursday, he received the Holy Viaticum, the Catholics' last rites for the sick and dying, the spokesman said.
This is a sign that his closest advisers fear the end of one of the longest papal reigns is fast approaching, says the BBC's David Willey in Rome.
And, for the first time during the Pope's current illness, the mood at the Vatican has suddenly shifted to one of pessimism, our correspondent says.
An update on the Pope's health is expected at 1030 GMT.
At the Vatican, Cardinal Camillo Ruini, the Pope's deputy for Rome who is traditionally charged with breaking the news of a papal death, called on believers to "intensify their prayers".
Traffic restrictions are being imposed around the Vatican and the Italian authorities are making plans to deal with a huge influx of pilgrims anxious to be present as this historic pontificate draws to its close.
Cardinals who have the duty to elect a new pope are beginning to arrive in Rome from all over the world, Italian media reports.
Pope stays put
The pontiff was provided with "all the appropriate therapeutic provisions and cardio-respiratory assistance", Mr Navarro-Valls' statement early on Friday said.
"This morning the condition of the Holy Father is very serious," the statement said, but the 84-year-old pontiff remained "conscious, lucid and tranquil".
It said the Pope had decided not to return to Rome's Gemelli hospital - but was being treated in his apartment by a team of four top consultants from the Catholic teaching hospital in Rome and his private doctor Renato Buzzonetti.
Groups of faithful have started gathering in St Peter's Square to pray for the Pope's speedy recovery.
At the Vatican, the mood is sombre
On Thursday afternoon, the Pope's temperature soared to 40C (104F), Italian media reported.
He developed breathing troubles and had difficulty swallowing as a result of the progress of Parkinson's Disease, an incurable condition from which he has been suffering for nearly a decade.
Prelates are openly expressing pessimism about the possibility of the Pope ever resuming the guidance of his one billion-strong Church.
The pontiff is being fed through a nasal tube to aid his recovery from throat surgery last month.
The Pope had appeared briefly at the window of his Vatican apartment on Easter Sunday to bless the faithful, but was not able to speak.
It was the first time during his 26-year pontificate that the Pope delegated the main Easter ceremonies to his cardinals.
He tried again to speak to the faithful 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 on his throat.