Church officials prepared the world and its 1.1 billion Roman Catholics for the end of the third longest papal reign in history - more than 26 years.
"The general conditions and cardio-respiratory conditions of the Holy Father have further worsened," said Vatican spokesman Joaquin Navarro-Valls.
"A gradual worsening of arterial hypotension has been noted, and breathing has become shallow. The clinical picture indicates cardio-circulatory and renal insufficiency. The biological parameters are notably compromised," Navarro-Valls said.
Rome Cardinal Camillo Ruini told a mass at the city's San Giovanni church that the Pope, who received the blessing for the dying after his health suddenly deteriorated overnight, "already sees and touches the Lord. He is already united with our sole Saviour".
Catholics flocked to churches to light candles and pray for the Polish churchman who became Pope in 1978 and revitalised the papacy. Groups of faithful gathered in the Vatican's vast St. Peter's Square, some gazing up at the papal apartments.
After weeks of worsening health, the Pope developed a high fever on Thursday caused by a urinary infection.
Recent images of a gaunt, pained John Paul, his body ravaged by Parkinson's disease and arthritis, contrast starkly with the sprightly Wojtyla who strode onto the world stage on October 16, 1978, and travelled the globe tirelessly to preach the Gospels.
The Pope came close to death before when a Turkish gunman shot him during a general audience in St. Peter's Square in 1981. He believes divine intervention saved him from death.
After a pope dies, cardinals from around the world are called to Rome to chose a successor at a conclave which starts in the Vatican's Sistine Chapel 15 to 20 days after the death.