Tens of thousands of mourners filed past the body of former Pope Benedict XVI lying in state in St Peter’s Basilica in the Vatican.
Vatican police said 65,000 people had paid their respects on the first day.
The doors of St Peter's Basilica opened for viewing by the public at just after 9am local time.
Public viewing lasts for 10 hours on Monday in St Peter's Basilica.
Twelve hours of viewing are scheduled for Tuesday and Wednesday before Thursday morning's funeral, which will be led by Pope Francis, at St Peter's Square. It will be the first time a current pontiff has officiated at the funeral of a former one.
German theologian Pope Benedict, who was 95, died on Saturday almost a decade after he resigned due to failing health. He was the first pope in 600 years to do so.
As daylight broke, 10 white-gloved Papal Gentlemen - lay assistants to pontiffs and papal households - carried the body on a cloth-covered wooden stretcher up the centre aisle of the basilica to its resting place in front of the main altar under Bernini's towering bronze canopy.
A Swiss Guard saluted as the body was brought in through a side door after Benedict's remains had been transferred from the chapel of the monastery grounds where the frail 95-year-old former pontiff died on Saturday morning.
The German cleric's longtime secretary, Archbishop Georg Gaenswein, and a handful of consecrated laywomen who served in Benedict's household, followed the van by foot in a silent procession towards the basilica.
By mid-morning the queue to enter the basilica snaked around St Peter's Square.
A few VIPs had a moment before the general public to pay their respects, including Italian premier Giorgia Meloni, the far-right leader who in the past has professed admiration for the conservative leanings of Benedict. Also viewing the body was President Sergio Mattarella.
The Vatican said only two nations - Italy and Germany - had official delegations with formal invites to the funeral, as Benedict was no longer head of state.
Filippo Tuccio, 35, arrived from Venice on an overnight train to view Benedict's body.
"I wanted to pay homage to Benedict because he had a key role in my life and my education. I arrived here at around 7.30am, after leaving Venice last night," he said.
"When I was young I participated in World Youth Days," he added, referring to the jamborees of young faithful held periodically and attended by pontiffs.
Mr Tuccio added that he had studied theology and "his pontificate accompanied me during my university years".
"He was very important for me, for what I am, my way of thinking, my values. This is why I wanted to say goodbye today."
The King led the tributes to Pope Benedict, recalling “with fondness” their meeting at the Vatican in 2009 and Pope Benedict’s visit to the UK the following year.
Charles wrote in a letter to Pope Francis: “His visit to the United Kingdom in 2010 was important in strengthening the relations between the Holy See and the United Kingdom.
“I also recall his constant efforts to promote peace and goodwill to all people, and to strengthen the relationship between the global Anglican Communion and the Roman Catholic Church.”
Archbishop of Westminster Cardinal Vincent Nichols, the head of the Catholic Church in England and Wales, hailed German-born Pope Benedict as “one of the great theologians of the 20th century”.
And he praised his UK trip, during which he met the Queen at Holyrood Palace in Edinburgh and made a speech at Westminster Hall.
Cardinal Nichols said in a statement: “We saw his courtesy, his gentleness, the perceptiveness of his mind and the openness of his welcome to everybody that he met.
“He was through and through a gentleman, through and through a scholar, through and through a pastor, through and through a man of God ? close to the Lord and always his humble servant.
“He will be remembered as one of the great theologians of the 20th century.”
Prime Minister Rishi Sunak tweeted: "Pope Emeritus Benedict XVI was a great theologian whose UK visit in 2010 was a historic moment for both Catholics and non-Catholics."
Pope Benedict XVI visits London
Labour leader Sir Keir Starmer posted: "His state visit in 2010 was a historic and joyous moment for Catholics in Britain."
The Vatican announced the death in a morning statement saying: "Pope Emeritus, Benedict XVI, passed away today at 9.34 in the Mater Ecclesiae Monastery in the Vatican."
He will lie in state in St Peter’s Basilica from Monday, with his funeral on Thursday.
In 2005, four million people went to Rome for the funeral of Benedict’s predecessor Pope John Paul II.