Mulugeta Aserate Kassa, an adviser at the Ethiopian embassy in London, told the BBC's Focus on Africa programme the patriarch had been "very healthy" on Wednesday, when he led a service in the Ethiopian capital, Addis Ababa.
He later fell ill and was rushed to hospital, said Mr Mulugeta, a close friend of the patriarch.
The patriarch died early on Thursday morning, he added.
Mr Mulugeta said he could not confirm the cause of death, but the patriarch had suffered from hypertension and diabetes.
He suffered a heart attack, according to opposition website Ethiopian Review.
Abune Paulos was chosen to lead the church in 1992 - a year after the overthrow of the communist government of Mengistu Haile Mariam.
"He brought the church in line to meet the challenges of the 21st Century," Mr Mulugeta said.
"He was a pragmatist at heart."
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