人群开始在梵蒂冈圣彼得广场上聚集，教皇将于周四卸职前做最后一次讲话。
The Pope will continue to wear his distinctive white cassock after retirement
Papal audiences are normally held inside a Vatican hall in the winter.
But such is the level of interest that the event is being held outdoors and 50,000 tickets have been requested. As many as 200,000 people may attend.
After Benedict XVI steps down on Thursday, he will become known as "pope emeritus（退休的）".
There has been no papal resignation since Pope Gregory XII abdicated in 1415.
The surprise announcement of Benedict's abdication has required the rules of electing a successor to be changed to allow the next pope to be chosen before Holy Week, which leads up to Easter.
On Wednesday, the Pope, 85, will be making one of his last public appearances - using his trademark white "popemobile" to greet pilgrims in St Peter's Square.
Organisers say there will be no traditional kissing of the pontiff's hand because of the sheer size of the expected crowd.
"He doesn't want to favour one or the other of the pilgrims," Vatican spokesman Federico Lombardi told the AFP news agency.
The BBC's Alan Johnston in Rome says that on Thursday the Pope will travel by helicopter to his summer residence at Castel Gandolfo, about 15 miles (24km) south-east of Rome. He will cease to be Pope at 20:00 local time.
On Tuesday, it emerged that Benedict would be known as "pope emeritus" and would retain the honorific "His Holiness" after his abdication.
He will also continue to be known by his papal title of Benedict XVI, rather than reverting to Joseph Ratzinger.
He will wear his distinctive white cassock without any cape or trimmings.
He will surrender his gold ring of office, known as the fisherman's ring, and his personal seal will be destroyed in the same way as when a pope dies.
Benedict will also give up wearing his specially-made red leather loafers, instead wearing brown shoes hand-made for him by a craftsman during a brief visit to Mexico last year, the Vatican said.