Tens of thousands of people are expected to attend a mass by Pope Francis in western Canada Tuesday, a day after his historic apology for the abuse of Indigenous children at Catholic-run schools. 

The 85-year-old pontiff is expected to deliver a homily in Spanish at Commonwealth Stadium in Edmonton, Alberta, in what will be one of the largest open-air events of his visit to Canada.

Organizers say more than 60,000 people are expected to attend the mass in Edmonton, which is being held under heavy security.

Knee pain has seen the Pope use a cane or a wheelchair in recent months, including in Canada, and he will greet crowds from his popemobile. 

In his first address Monday, to a gathering of Indigenous people in the community of Maskwacis, south of Edmonton, he offered a long-awaited apology to Canada's First Nations, Metis and Inuit people for the "evil" done over decades of abuse in schools.

"I am sorry," he said, adding: "I humbly beg forgiveness for the evil committed by so many Christians against the Indigenous peoples."

He cited the "cultural destruction" and the "physical, verbal, psychological and spiritual abuse" of children over nearly a century at the schools. 

From the late 1800s to the 1990s, Canada's government sent about 150,000 children into 139 residential schools run by the Church, where they were cut off from their families, language and culture.

Many were physically and sexually abused, and thousands are believed to have died of disease, malnutrition or neglect.