A nun stands in prayer in front of St. Peter's Basilica in the Vatican on Tuesday as Cardinals hold a pre-conclave mass inside as they prepare to vote to choose Pope Benedict XVI’s successor. 

 Cardinals enter St. Peter's Basilica in the Vatican on Tuesday for their pre-conclave mass prior to voting to choose Pope Benedict XVI’s successor. 

 Guards stand watch at St. Peter's Basilica in the Vatican.

 A cardinal enters St. Peter's Basilica.

 A guard stands watch in one of the entrances to St. Peter's Basilica.

 A nun stands in prayer in St. Peter's Square in the Vatican on Tuesday as the College of Cardinals hold a pre-conclave mass inside the basilica as they prepare to vote to choose Pope Benedict XVI’s successor.  Pilgrims gathered outside to watch the mass on several big screens.

 A nun holds rosary beads as she stands in prayer in St. Peter's Square.

 A man stands in St. Peter's Square in the Vatican on Tuesday as rain begins to fall on pilgrims and tourists who gathered in the square on the first day of the conclave.

 A couple holds each other in St. Peter's Square in the Vatican.

 A pilgrims kneels in the rain in St. Peter's Square as the College of Cardinal's hold mass at St. Peter's Basilica.

 A woman prays in St. Peter's Square.

Tourists and pilgrims stand in St. Peter's Square in the Vatican on Tuesday as rain begins to fall on the first day of the conclave, where the College of Cardinals will vote for the first time to choose Pope Benedict XVI’s successor.  Read more about the conclave in the Pittsburgh Tribune-Review.