The unscripted moment capped a ceremony fraught with symbolism for thousands of residential school survivors who sat in sombre silence as Francis said how "deeply sorry" he was for the Catholic church's role in Canada's abusive residential school system.

It was a long-awaited apology on First Nations soil.

"It was very emotional. I don't know. Do we celebrate? It was powerful to hear the leader of the Catholic Church ask us ... to forgive him," Maureen Belanger, a residential school survivor who was in the venue, told CBC TV.

"At the same time, you can't forget all the spirits that are not at rest."

The pope spoke to about 2,000 people assembled around him in an open-air, circular auditorium while more watched on large screens from a distance.

Many were survivors of the residential school system that, over more than a century, forcibly separated more than 150,000 indigenous children from their families and subjected many to starvation, beatings and sexual abuse in what Canada's Truth and Reconciliation Commission called "cultural genocide."

Participants carried a 50-metre-long red banner with the names of thousands of missing indigenous children through the auditorium before the speech.

Some wore indigenous regalia while others wore orange shirts to mark the legacy of the residential school system and children who never came home from the institutions.

Some watched intently as the pope spoke, while others leaned on each other. Some wept.

When the pope's translator read out how "deeply sorry" the pontiff was, people cheered.

The pope begged for forgiveness even as he said "there were many outstanding instances of devotion and care for children" at the schools.

After the pope spoke, Chief Wilton Littlechild placed a feather headdress on the pontiff's head as the crowd cheered. Soon after, the indigenous woman in regalia sang the anthem in Cree as the pope watched.

That moment was unscripted, and happened "organically," said an official with the papal visit.

After she sang the woman cried out to the pope in Cree.

"She was telling him that this (land) was a pure place - a clean place - prior to the settlements," said Ermineskin Nation Chief Randy Ermineskin.

Shortly after the pope stopped speaking, an unidentified woman yelled: "Repudiate the doctrine of discovery! Renounce the papal bulls!"

The papal bulls were 15th-century edicts that justified taking indigenous land, and many indigenous leaders have called on the pope to formally rescind them.

It was emotional viewing for residential school survivor Ruth Roulette, from Long Plain First Nation in Manitoba. She watched Monday's ceremony and apology at home with her 11-year-old great-grandson Cedrik.

"I hope he never, ever has to experience what we did," she said.

The pope's visit brought back painful memories, she said: "Things that I thought I buried."

Her husband, also a survivor, could not watch.

"He still hasn't recovered," she said.

Phil Fontaine, a residential school survivor and former Assembly of First Nations National Chief who was at the event, said the apology was "a special moment for survivors."

"For those that desperately needed to hear the words 'I'm sorry' or something to that effect, I think it was an important day for them," said Fontaine.

(Additional reporting by Tim Johnson, Writing by Anna Mehler Paperny, editing by Deepa Babington)