Crowds of tens of thousands of people are filling the streets around the Vatican, queuing to see the body of Pope John Paul II lying in state.
The Vatican said the Pope's body had not been embalmed
Catholic cardinals have meanwhile been meeting to arrange the Pope's funeral and the election of his successor.
A spokesman said tolling bells, as well as the traditional white smoke, would announce a new Pope had been chosen.
World leaders, pilgrims and tourists are expected to attend Friday's funeral for the Pope, who died last week.
Streets around the Vatican have been crammed with people eager to get a last glimpse of one of the longest-reigning leaders of the Roman Catholic Church.
Some had travelled and waited hours to view his body in St Peter's Basilica, others came early on Tuesday before work.
A Vatican spokesman said no date had been set for the grand gathering of cardinals, known as a conclave, that meets to pick a new Pope.
He said they had yet to read a testament left behind by the Pope, which may contain the name of a "secret" cardinal he appointed two years ago.
Such appointments are typically made when the church wants to protect the identity of a cardinal who belongs to a country where the faith has suffered persecution.
Several unprecedented additions appear to have been made to the traditions that attend the election of a Pope.
A Vatican spokesman said the naming of the new Pope will be announced by bells as well as the customary plume of white-smoke, which he said, was not a clear enough signal.
200 international leaders to attend
Two million mourners predicted in Rome
Hundreds of millions to watch on TV
While the Vatican traditionally uses white smoke to tell the world a new Pope has been elected, the plumes have been confused in the past with black smoke, which is released to indicate no decision has been reached after a round of voting.
A Vatican spokesman also said the conclave will be held throughout the whole city-state for the first time in its history.
This means cardinals will be free to move between different parts of the complex, although they will still be barred from contact with the outside world.
Hope for new pope
Officials had planned to close St Peter's basilica, where the Pope's body is being kept, for three hours each night to allow for cleaning.
But in the event, the basilica stayed open until 0300 (0100 GMT) on Tuesday, and reopened barely an hour and a half later for the public to be readmitted.
The wait was so long that some elderly people fainted, while others in the crowd sang to pass the time.
When they entered the basilica, some mourners took pictures of the Pope's body with their mobile phone cameras.
Sister Gladys, a nun from Manila, told the BBC: "It is very important for me to come and pay my last respects to this man who I love very much.
"I am praying that there will be a Pope like him, a Pope who loves everybody, who speaks about God not only by his words but also by his actions - a Pope something like Pope John Paul II."
Flood of visitors
About 200 foreign dignitaries, including US President George W Bush, will attend the funeral.
The gathering will be one of the largest of world leaders in history and Rome is expecting a total of up to two million mourners to take to the city's streets on Friday.
John Paul II died after suffering from heart and kidney problems and unstable blood pressure and he had been increasingly ill and frail for years.
Born Karol Wojtyla, he became Pope in 1978, taking a conservative stand on issues like contraception and "liberation theology".
He was the most widely travelled pontiff and visited more than 120 countries during his 26-year papacy.