Tens of thousands of mourners gathered on Thursday for the funeral of former pope Benedict XVI at the Vatican, led by his successor Pope Francis in an event unprecedented in modern times.

The body of the German theologian, who in 2013 became the first pontiff in six centuries to resign, was laid out in a simple cypress coffin in front of St Peter's Basilica, where his remains will later be laid in the crypt.

For the first time in modern history, the proceedings were led by a sitting pope, Francis, who will deliver the homily.

On either side of the coffin were seated red-clad cardinals and dignitaries from around the world, among them heads of state, including German Chancellor Olaf Scholz and Italian Prime Minister Giorgia Meloni.

Tens of thousands of members of public also attended, many of them queuing before dawn to pay their respects to Benedict, who died last Saturday aged 95.

"Benedict is a bit like my father, so I had to pay homage to him," said Cristina Grisanti, a 59-year-old from Milan, who hailed the former pope's "purity, his candour, his mildness".

Many Germans were also in the crowd, paying tribute to Germany's first pope in 1,000 years, whose funeral is being marked back home by the ringing of church bells across the country.

"We owe him so much. We want to show that we stand behind him," said Benedikt Rothweiler, 34, who came from Aachen with his family.

"We actually know too little about Benedict. He always accepted everything the way God wants it. This is a good example for us humans."

An estimated 195,000 people have already paid their respects during three days of lying in state at the basilica, the Vatican said, while up to 100,000 were expected for Thursday's funeral.

Benedict will be interred in a tomb in the crypt beneath the basilica, where John Paul II's body lay before it was moved for his beatification in 2011. He was made a saint in 2014. (AFP)