He said the immigration officials "imagine I want to stay back illegally".

Dr Ogwell was due to attend the World Health Summit in the German capital, Berlin, that began on Sunday and finishes on Tuesday.

Following the incident, he said his "attendance was in doubt" despite having a visa and an invitation to the summit.

On Monday he tweeted: "I’m safely back in Africa home to the most resilient human beings I know."

He also shared some lessons:

"A visa often means little when you are carrying an African passport - regular or diplomatic"
"Don’t reduce your dignity to fit someone else’s prejudices - you’ll be feeding a wolf that will one day devour you"
"Very many decent people are mistreated everyday and don’t have the microphone to let the world know - I empathize with ALL of you"
"Home is best - Mama Afrika remains my refuge (and 1.5 billion others!) so I must take care of her. And so should you."
Frankfurt airport apologised for the Saturday incident.

"As an international airport, we welcome all passengers and do not tolerate any form of discrimination or racism. We would like to have more information about this incident," it tweeted after Dr Ogwell had shared his ordeal at the airport.