People hug in front of a makeshift memorial in Kamloops, British Columbia, Canada. AFP

The revelation once again cast a spotlight on a dark chapter in Canada's history, and revived calls on the Pope and the Church to apologize for the abuse suffered at the schools, where students were forcibly assimilated into the country's dominant culture.

"As of yesterday, we have hit 751 unmarked graves" at the former Marieval boarding school in Saskatchewan province, Cowessess First Nation Chief Cadmus Delorme told reporters.

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"This is not a mass grave site. These are unmarked graves," he said, adding that each plot would be assessed in the coming weeks to determine the number of victims buried at the site.

Delorme said the graves -- found through ground-penetrating radar mapping -- may at one time have been marked, but "Catholic Church representatives removed these headstones," adding that doing so is a crime in Canada and they were treating the site "as a crime scene."

"Without a doubt, they were trying to cover up the amount of children that were being mistreated and killed in those institutions," Bobby Cameron, head of the Federation of Sovereign Indigenous Nations in Saskatchewan, told broadcaster CBC.

Thundersky Justin Young (left) and Daryl Laboucan drum and sing healing songs to honour the 215 children whose remains have been discovered in Kamloops, British Columbia, Canada. File/AFP

"We had concentration camps here," Cameron said separately at a news conference. "Canada will be known as the nation who tried to exterminate the First Nations."

The exact number of victims will not be known for weeks because the radar mapping equipment has a margin of error and graves may contain more than one set of remains, said Delorme.

Excavations at the Marieval school, about 150 kilometers (90 miles) east of the provincial capital Regina, began at the end of May, after the discovery of the remains of 215 schoolchildren at another such former school in British Columbia.

That first find at the Kamloops school triggered excavation work near several former institutions for indigenous children across Canada, with the assistance of government authorities.

Until the 1990s, some 150,000 Native American, Metis and Inuit children were forcibly recruited into 139 of these residential schools across Canada, where they were isolated from their families, language and culture.

Many were subjected to ill-treatment and sexual abuse, and more than 4,000 died in the schools, according to a commission of inquiry that concluded Canada had committed "cultural genocide."

Cameron described the finding as "a crime against humanity."

"The only crime we ever committed as children was being born indigenous," he said.

Prime Minister Justin Trudeau called the findings at both Kamloops and Marieval "a shameful reminder of the systemic racism, discrimination and injustice that indigenous peoples have faced -- and continue to face -- in this country."

"Together, we must acknowledge this truth, learn from our past and walk the shared path of reconciliation, so we can build a better future," he said.

Agence France-Presse