The doors of St. Peter's Basilica, where the former pope's body lies, were opened to people past 9 a.m. (0800 GMT; 3 a.m. EST) when the first faithful entered.
His body, dressed in red and gold liturgical vestments, was placed on a simple dais, with two Swiss guards standing on either side as the faithful walked by.
Public viewing lasts for 10 hours on Monday in St. Peter's Basilica. Meanwhile, on Tuesday and Wednesday, the viewing will happen for 12 hours.
On Thursday, Pope Emeritus Benedict XVI's funeral will be held, which will be led by Pope Francis at St. Peter's Square.
The frail, 95-year-old Benedict died Saturday morning in the Vatican monastery where he had lived since his retirement in 2013, when he became the first pontiff to resign in 600 years.
Security officials expected at least 25,000 people to pass by the body on the first day of viewing.
During his eight-year papacy, Benedict, a methodical, shy and very private German, had a hard time filling the shoes of the charismatic John Paul, inviting constant comparison in the media and among the faithful of the 1.3 billion member Church.
