Tens of thousands of people and more than 100 delegations have come together to pay respects to the late leader of the Catholic Church, who will be laid to rest in the Italian capital on Saturday.

After lying in state for three days, Pope Francis will be laid to rest on Saturday following a funeral ceremony at St Peter's Square in Vatican City.

A funeral Mass was held outdoors on Saturday morning, with heads of state, including US President Donald Trump and his Ukrainian counterpart Volodymyr Zelenskyy, slated to attend.

Thousands of mourners, many of whom travelled from across the world to attend, began to gather in the public square as early as 5 am, well in advance of the public Mass five hours later.

As many as 250,000 people were in attendance, according to the Holy See.

The late pontiff will be transported to be buried in the Basilica of Santa Maria Maggiore, or Saint Mary Major, one of the four papal basilicas in Rome.

Pope Francis revised and simplified the Vatican's rites ahead of the funeral, which will see presidents and royals attend while convicts and migrants usher him to his final resting place.

The pope hoped the ceremony would reflect the priorities of his papacy, where he aimed to emphasise his role as a mere priest rather than a powerful religious leader.