Pope John Paul II, the third longest-serving pontiff in history, has died at the age of 84.
The Pope died in his private apartment at the Vatican at 2137 local time (1937 GMT) on Saturday, surrounded by his closest Polish aides.
Many thousands of people gathered in Rome's St Peter's Square to pay tribute to the pontiff, while church bells throughout the city began tolling.
The Pope had suffered worsening health problems including a heart condition.
Vatican spokesman Joaquin Navarro-Valls said a mass was held at his bedside by senior officials before his death, at 2000.
The Pope then received the Viaticum, a Catholic rite for the sick and dying.
The Italian government has declared three days of mourning.
The Pope is to lie in state in St Peter's Basilica from Monday afternoon, the Vatican said.
News of the Pope's death was received with intense emotion
The funeral date has not been set but it is not expected before Wednesday.
Pope John Paul II died after suffering from heart and kidney problems and unstable blood pressure.
Minutes after his death, the Vatican issued a brief statement to confirm the news, adding that procedures to be carried out in the event of the death of the Pope had been set in motion.
The Pope's death was immediately announced to the crowds gathered in St Peter's Square.
THE POPE'S LIFE
1920 - Born near Krakow, Poland
1964 - Archbishop of Krakow
1978 - Elected first non-Italian Pope for 450 years
1981 - Assassination attempt
2002 - Final visit to homeland
The news was met with long applause, an Italian sign of respect, followed by several minutes of silence as the crowd took in the news.
"Our Holy Father John Paul has returned to the house of the Father," senior Vatican official Archbishop Leonardo Sandri said.
The BBC's Peter Gould, at the Vatican, says people in the square stood in groups, comforting one other.
In the Pope's native Poland, people fell to their knees and wept as the news reached them.
Tributes have been coming in from political and religious leaders in other parts of the world.
US President George W Bush said the world had lost a champion of freedom.
UN Secretary-General Kofi Annan said he was deeply saddened by the death of a Pope whom he described as a tireless advocate of peace.
The Cardinal Chamberlain of the Roman Catholic Church, Eduardo Martinez Somalo, is now in charge.
He has to seal the papal apartments and summon the cardinals from around the world to elect the Pope's successor.
The cardinals, many of whom are already on their way to Rome, must meet no more than 20 days after the Pope's death to choose a successor. A preliminary meeting has been arranged for Monday morning.
The Pope's condition deteriorated suddenly on Thursday night with a high fever caused by an infection of the urinary tract.
He had been suffering from breathing troubles, exacerbated by the progress of Parkinson's Disease, an incurable condition from which he had been suffering for nearly a decade.
He appeared briefly at the window of his Vatican apartment on Easter Sunday to bless the faithful, but was unable to speak.
It was the first time during his 26-year pontificate that the Pope had delegated the main Easter ceremonies to his cardinals.
Polish-born Karol Wojtyla became Pope in 1978, taking a conservative stand on issues like abortion and contraception.
He was the most widely travelled pontiff and visited more than 120 countries during his 26-year papacy.