They began queueing before dawn to view the German theologian's body, which was transferred early Monday from the monastery in the Vatican grounds where he died Saturday aged 95.
"I arrived at 6:00 am, it seemed normal to come and pay homage to him after all he did for the church," said an Italian nun, sister Anna-Maria, near the front of the queue that snaked around the edge of the vast St Peter's Square.
Many of those filing past took pictures on their smartphones of the body, which was dressed in red papal mourning robes with a gold-edged miter on his head, while some prayed or made the sign of the cross.
"The atmosphere is very intimate," Francesca Gabrielli, a pilgrim from Tuscany in central Italy, told AFP inside the basilica.
She said Benedict was "a great pope, profound, unique."
Benedict led the Catholic Church for eight years before becoming the first pope in six centuries to step down in 2013, citing his declining and physical health.
His successor, Pope Francis, will lead the funeral on Thursday in St Peter's Square before his remains are placed in the tombs beneath the basilica.
Papal deaths usually trigger the calling of a conclave of cardinals to elect a successor, but this time Francis remains in post, and will lead proceedings.
The Vatican has yet to release details of the guest list, beyond saying that it will include delegations from Italy and Benedict's native Germany.
The last papal funeral, of John Paul II in 2005, drew a million faithful and heads of state from around the world, although Benedict was a more divisive figure.
A brilliant theologian, he alienated many Catholics with his staunch defence of traditional values and as pope struggled to impose his authority on the church as it battled a string of crises, including over clerical sex abuse.
His successor cuts a very different figure, an Argentine Jesuit who is most at home among his flock and has sought to forge a more compassionate church.
Francis, 86, has raised the prospect that he might follow Benedict's example and step down if he became unable to carry out his duties.
In July, suffering knee problems that have forced him to rely on a wheelchair, he admitted he needed to slow down or think about stepping aside.
Last month, Francis revealed he had signed a resignation letter when he took office should poor health prevent him from carrying out his duties.