Pope Francis is slated to arrive in Canada on Sunday to apologize for historical abuse inflicted upon indigenous children by leaders at Catholic-run residential schools.

He is scheduled to land in Alberta at around 11:20 a.m. to kick off what he calls a "pilgrimage of penance." Francis will make two more stops in Canada, also visiting Quebec City and Iqaluit.

Free tickets for the event were claimed shortly after being made available, according to the trip's organizer, the Canadian Conference of Catholic Bishops.

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Pope Francis will land in Alberta Sunday morning to kick off what he calls a "pilgrimage of penance." ((Photo by Stefano Costantino/SOPA Images/LightRocket via Getty Images))

Thousands of the tickets were reserved for indigenous survivors of the residential schools. Indigenous leaders in Treaty 6, the location Francis is visiting in Alberta, have said they were overwhelmed with requests from survivors who wish to attend the event.

More than 150,000 indigenous children were separated from their families and brought to residential schools between 1881 and 1996. A number of children were allegedly starved, beaten and sexually abused.

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Pope Francis leaves after a meeting with a group of Cuban youth in Havana, Cuba, Sunday Sept. 20, 2015. Pope Francis met with Fidel Castro on Sunday before finishing the day with a vespers service in Havana's cathedral, and a meeting with the youths. In the background at right is Cardinal Sean Patrick O'Malley. (AP Photo/Ramon Espinosa)

Pope Francis will make three stops in Canada, visiting Alberta, Quebec City and Iqaluit. (AP Photo/Ramon Espinosa)

Canadian leaders have been aware of the many children dying at the schools since 1907, but the incidents garnered more attention following last year's discoveries of what appeared to be unmarked graves at or near former residential schools.

Pope Francis then apologized for the role the Catholic Church played in the schools earlier this year when indigenous delegates visited the Vatican before heading to Canada

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Pope Francis addresses journalists during the traditional press conference on his flight back to Rome, following a two-day visit at Fatima, Portugal, Saturday, May 13, 2017. Pope Francis added two Portuguese shepherd children to the roster of Catholic saints Saturday, honoring young siblings whose reported visions of the Virgin Mary 100 years ago turned the Portuguese farm town of Fatima into one of the world's most important Catholic shrines. (Tiziana Fabi/Pool Photo via AP)

More than 150,000 indigenous children were separated from their families and brought to residential schools between 1881 and 1996, and a number of these children were allegedly starved, beaten and sexually abused. (AP)

But some survivors and indigenous leaders say an apology is not enough.

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Demands have been made for financial compensation, the return of indigenous artifacts, the publication of school records, support for an accused abuser's extradition and the repeal of a doctrine from the 15th Century that defended colonial dispossession of indigenous people through a papal bull, or edict.

Reuters contributed to this report.