"Absolute arrogance and contempt is the only basis for this arrest," Kagame said in a speech to parliament.
"They must have mistaken him for an illegal immigrant. The way they treat illegal immigrants is the way they treat all of us. Black people have become targets for shooting practice," he said. "We cannot accept that people treat us this way just because they can."
President Paul Kagame's speech came on the same day that spy chief General Karenzi Karake is due to appear in court in London following his arrest under a Spanish indictment accusing him of involvement in the murder of nine Spanish aid workers.
"They want to mask their responsibility by saying it's not us, it is savages of Africa who killed each other," he said. "Since when did we become killers? Who has killed more Africans than they have? They have killed millions of Africans."
Kagame questioned the right of Britain to act on the Spanish indictment, issued in 2008, that accuses Karake of mass ethnic killings of Hutus in the wake of the 1994 genocide and of orchestrating the murder of Spanish aid workers in 1997.