Pope Francis surprised 150 homeless persons on Thursday when he greeted them during a special tour of the Sistine Chapel and Vatican Museums.

The guests were also treated to dinner, and used an entrance usually designated for employees and prelates.

"This is everyone's house, it is your house," Francis told the homeless, according to a Vatican spokesperson. No media were allowed during the unique tour.

A statement from the Vatican said the homeless people frequent the area around St Peter's Basilica, and usually only see the outside of the iconic buildings.

The Pope's almoner, Bishop Konrad Krajewski, organised the visit, which called for the chapel to close to the public earlier than usual.

The visit by Francis is one example of many in which the pontiff has demonstrated a concern for the downcast.

During a sermon at St Peter's Basilica last month, the pontiff warned against having a "prejudiced mentality" that marginalises others, and used the biblical story of Jesus healing a leper as an example of how Christians should treat society's outcast.

"Jesus responds immediately to the leper's plea, without waiting to study the situation and all its possible consequences," Francis told hundreds of cardinals and bishops.

"For Jesus, what matters above all is reaching out to save those far off, healing the wounds of the sick, restoring everyone to God's family."

On Saturday, Francis visited the Giuseppe Salvia Detention Center in Poggiorale, Italy, and had lunch with 90 convicts. The inmates, who were chosen at random, included 10 men from the prison's ward for gay, transgender, and HIV-positive convicts.

The Religion News Service reported that Francis insisted on having lunch with the prisoners, although it was not originally on his Naples itinerary.