Italian officials and Vatican clerics have been paying their respects to Pope John Paul II, whose body is lying in state in the Vatican.
Italy is to hold three days of mourning for the Pope
Mourners, including Italian Prime Minister Silvio Berlusconi, filed past the Pope shown on television dressed in a crimson robe and a white mitre.
Earlier, tens of thousands of mourners in St Peter's Square in Rome attended an open-air Mass in honour of the Pope.
John Paul II died on Saturday after a long illness, aged 84.
The Pope's body was laid out on a plinth in the Vatican palace's Clementine Hall, at the start of nine days of official mourning in the city state.
The pontiff's head rested on a pillow, his arms were folded and a bishop's staff was tucked under his left arm. A crucifix stood to one side and a tall candle burned on the other.
His body will be taken to St Peter's Basilica on Monday for a public viewing.
Officials believe up to a million people could come to Rome to pay their respects.
The funeral date has not been set, but it is not expected before Wednesday.
The Vatican issued the Pope's death certificate on Sunday, saying he died from septic shock and irreversible heart failure.
Sunday's Requiem Mass on the steps of St Peter's Basilica was celebrated by Cardinal Angelo Sodano, regarded as the second highest official in the Vatican hierarchy.
Applause rang out in the crowd when Cardinal Sodano prayed for the pope's soul at the start of the service.
"We entrust with confidence to the risen Christ, Lord of life and history, our beloved John Paul II who for 26 years guided the universal church as the successor of Peter," he said.
The Pope's last message to pilgrims was read out: "Love converts hearts and gives peace," it said.
Catholic communities around the world have also been grieving for the pontiff:
- In the Pope's native Poland, bells rang out across the country and night-long vigils were held
- Brazil, home to the world's largest Catholic population, has declared seven days of national mourning
- Government offices in the Philippines, Asia's largest Catholic country, have been told to fly the flag at half-mast. A period of mourning will be held from Monday until the Pope is buried
- In communist Cuba, three days of official mourning are being held
- On Indonesia's Nias island, survivors of Monday's huge earthquake gathered outdoors to mourn the Pope in their first Mass since the tremor.
In the UK, a spokesman for Prime Minister Tony Blair said he had postponed plans to announce a general election on Monday afternoon, in order to attend a service in memory of the Pope.
The Pope died in his private apartment at the Vatican at 2137 local time (1937 GMT) on Saturday, surrounded by his closest Polish aides.
An announcement was met with long applause - an Italian sign of respect - from the pilgrims in St Peter's Square, followed by several minutes of silence as the crowd took in the news.
Up to 100,000 people then gathered in the square below the Pope's apartment for a candlelit vigil during which many prayed and wept openly.
The BBC's Peter Gould, at the Vatican, reports that people in the square stood in groups, comforting one other.
"It is as if our father has died. There is a deep sense of loss and emptiness," said 28-year-old Mario Deluca.
In Poland, some people fell to their knees and wept when the news reached them.
It could be weeks before Church leaders elect a successor.
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 Cardinal Chamberlain of the Roman Catholic Church, Eduardo Martinez Somalo, is now in charge of the day-to-day running of the Vatican's affairs.
Earlier on Sunday, he sealed the papal apartments, which will not be reopened until the cardinals elect a new pope.
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 the successor.
A preliminary meeting has been arranged for Monday morning.