A major police operation is unfolding at Yongah Hill Detention Centre in Northam, with dozens of officers, including the riot squad, on site.

At least one man has been arrested, and it's understood someone has been stabbed and a fire was lit inside the facility.

Police have yet to release any details but 9 News Perth reporter Kelly Haywood told Gareth Parker on 6PR Breakfast the incident appeared to unfold last night and she saw an ambulance leaving when arriving on the scene a few hours ago.

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