A man became ill while in custody at Forth Banks station in Newcastle on the same day he was arrested, police said. He was pronounced dead at 3.45pm on Thursday.
Assistant Chief Constable Jo Farrell said: “This death within our custody is a tragic incident.
“We immediately referred the death to the IPCC (Independent Police Complaints Commission) for independent investigation, as we would with any death in police custody, and we are assisting them fully.”
She added: “Northumbria Police takes its commitment to the welfare of prisoners extremely seriously. This death comes on the same day the IPCC released their annual statistics on deaths during or following police contact.
“Northumbria Police’s statistics were included in this report and as a force we welcome this in the interests of openness and transparency.”
Figures from the IPCC showed that there were 17 fatalities during or after detention in England and Wales in 2014/15 – six more than the previous year and the highest annual total since 2010/11.
There were also 69 apparent suicides after release – 50% higher than in 2010/11.
Home Secretary Theresa May has launched a review into deaths in police custody, saying they have the potential to “dramatically” undermine relationships between the public and police.
The 17 deaths recorded in or after police custody in the last financial year is “broadly in line with the average” over the last six years, the IPCC said. Last year’s total of 11 was the lowest since recording began in 2004/5.