Faithful left flowers Saturday near a new, simple marble slab in St. Peter's Square which marks the precise spot where Pope John Paul II was shot in an assassination attempt exactly 25 years ago.

The white stone is decorated with the date, in Roman numerals, of the May 13, 1981, shooting by a Turkish gunman which gravely wounded John Paul while he was being driven in his popemobile during a general audience in the square.

The stone is set flat into the square, replacing some cobblestones.

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His successor, Pope Benedict XVI, announced the placement of the stone in a message read to thousands of faithful during a Mass in St. Peter's Basilica to mark the attack anniversary.

Participants were also honoring the Madonna of Fatima whose feast day is celebrated on May 13. John Paul credited his survival from the shooting to what he said was the intercession of the Madonna.

Earlier in the day, thousands gathered near the Tiber for the start of a procession to the square. Above the crowd, a helicopter carried a Fatima statue with a bullet embedded in it as a memento of the assassination attempt. The statue was later carried, on a bed of flowers, into the basilica.

May 13 also is the Catholic feast day commemorating what three shepherd children in Fatima, Portugal, said was the first time the Virgin Mary appeared to them in 1917.

The statue is usually kept in a shrine in Fatima, Portugal. One of the bullets that struck John Paul was later embedded in the statue.

In Benedict's message, the pope said the stone had been placed on the exact spot to help people "remember from now on that dramatic event."

Benedict added a wish for peace for all: "in hearts, in families and among peoples."

The late pope was particularly devoted to the Virgin of Fatima and visited the shrine as part of his pilgrimages.

"Twenty-five years ago, our beloved Pope John Paul II shed his blood," said Rome Cardinal Camillo Ruini, who celebrated the Mass. "May he be soon beatified," the cardinal added, drawing applause from the faithful.

Beatification is the last formal step before possible sainthood in the Catholic church.