In Poland Roman Catholics have been filing into churches to pray for their country's most famous son, while small groups of faithful huddled together in the Vatican's vast St Peter's Square, gazing up at the papal apartments.
"This will be a day of unity for human beings around the world," said Elzbieta Zak, a Pole who has lived in Rome for 20 years and was praying alongside two nuns.
In churches in Krakow, Poland, where the Pope studied and served as archbishop, at least twice the usual number of faithful attended early Mass on Friday.
"I didn't sleep at all last night and I decided to come and pray again this morning before I went to work," said Teresa Ptak at St. Florian's church, where Karol Wojtyla once worked before becoming Pope.
Cardinal Cormac Murphy-O'Connor, head of the Catholic Church in England and Wales, said fellow Christians and countless others were thinking of him as he was "approaching the last days or hours of his long pontificate".
The cardinal urged people to pray for the Pope, who has led the world's one billion Roman Catholics for 26 years.He said: "We are all thinking and praying for the Holy Father, Pope John Paul II, at this time.
Millions of Catholics in Asia packed churches and held vigils.
"We are all very sad about his failing health," said President Gloria Macapagal Arroyo in the Philippines, where four out of five people are Catholics. Underscoring the sombre mood, Italian political parties halted campaigning for regional elections this weekend and Prime Minister Silvio Berlusconi cancelled all appointments.
The Pope has been seriously ill for most of the past two months and failed to recover from recent throat surgery aimed at helping him breathe.
Doctors stayed at his side through the night. Senior clergy rushed to the Vatican this morning as his health worsened.
Italian media reported that the Pope's temperature leapt to around 40 C (104 F) yesterday afternoon and his blood pressure plunged, a day after doctors had inserted a feeding tube through his nose and into his stomach to boost his fading strength.
The third-longest-serving pope in history spent 28 days in hospital in two periods in February and March after suffering breathing crises.
Once dubbed the "Great Communicator", he has been unable to speak in public since he last left hospital on March 13, with a tube to help him breathe inserted in his windpipe.