The body of the retired pontiff will be displayed in an open casket for three days before his funeral on Thursday.
Tens of thousands flocked to the Vatican on Monday to pay their final respects to Pope Benedict XVI, who died on Saturday at the age of 95.
More than 65,000 people filed pasted the body of the retired pontiff, as his head rest on two crimson pillows in St. Peter's Basilica.
A long queue formed around St Peter's Square in the tiny city-state in Italy at dawn on Monday, as Benedict's lying-in-state began.
It comes ahead of his funeral on Thursday.
Italian security officials had expected at least 25,000 to 30,000 people to come and pay their respects to the former pope, who shocked the world by retiring in 2013.
The body of Joseph Ratzinger -- as he was known before becoming pope -- has so far remained in the small private chapel of the monastery where he lived since retiring. It is located in the heart of the Vatican gardens.
On Sunday, the Vatican released the first photos of the body, lying on a catafalque, dressed in red - the colour of papal mourning - and wearing a white mitre adorned with a golden braid.
A crucifix, a Christmas tree and a nativity scene are visible in the background.
The transfer of his body to St. Peter's Basilica, the largest Catholic church in the world that can accommodate tens of thousands of worshippers, took place at dawn on Monday.
The basilica, a masterpiece of architecture combining Renaissance and Baroque styles, was completed in 1626.
It is also one of the holiest places in Christianity since it houses the tomb of Saint Peter, the first bishop of Rome.
On Sunday, Pope Francis once again paid homage to "beloved" Benedict XVI, "that faithful servant of the Gospel and of the Church".
A brilliant theologian and fervent guardian of dogma, Benedict XVI, who had resigned in 2013 because of his declining health, died peacefully on Saturday morning.
The funeral ceremony will be "solemn but sober", according to the Vatican. It will take place on Thursday from 09:30 in Saint Peter's Square, where the funeral of his predecessor John Paul II attracted a million people in 2005. Germany's first pope in modern history will then be buried in a crypt in the basilica.
Benedict XVI's last words, spoken in Italian hours before his death on Saturday were: "Lord, I love you," his private secretary, Bishop Georg Gänswein, reported to Vatican News.