Preceded by papal officials, Pope John XXIII's body - dressed in a white tunic, a red velvet cape and red hat - was wheeled through the square so that he could be seen by as many people as possible before being reinterred.
His body went on display in a glass coffin in St Peter's Square in the Vatican during a mass conducted by Pope John Paul II.
His face was covered with a thin layer of wax, but his features are recognisable from old photographs.
Preceded by papal officials, Pope John XXIII's body - dressed in a white tunic, a red velvet cape and red hat - was wheeled through the square so that he could be seen by as many people as possible before being reinterred.
The transparent coffin was then taken inside the St Peter's Basilica, where thousands of pilgrims were expected to file past to pay their last respects.
His final resting place will be in a side chapel near the main altar in which St Peter, the first Pope, is believed to be buried.
No miracle
Vatican officials say that when they opened Pope John XXIII's coffin in January, in preparation for its transfer from a Vatican crypt, the body was found to be remarkably well preserved.
Popes by tradition are not embalmed but John XXIII's body was treated with formalin before it was laid in state.
The late pontiff, who died in 1963, had also been placed in a multi-layered lead coffin to keep out oxygen and slow the decomposition process.
John XXIII is particularly remembered for introducing the vernacular to replace Latin in church masses and for creating warmer ties between the Catholic Church and the Jewish faith.
He also called the Second Vatican Council, which continued under his successor Paul VI.
On his way to sainthood
It marked a major turning point in the Roman Catholic Church's history by redefining the nature of the church, giving bishops a greater say in church affairs and increasing lay participation in liturgy.
John XXIII has a huge following in Italy, where he is known as Il Papa Buono, the good pope.
Vatican insiders believe that he could soon be canonised.
The Vatican said in January last year that a miracle attributed to him actually took place, opening the final door for his beatification by John Paul II less than nine months later.
Under Roman Catholic tradition, beatification is a critical step towards sainthood.