Rwanda's intelligence chief Karenzi Karake, who is wanted in Spain for war crimes, has been arrested in London.
Gen Karake, 54, was arrested at Heathrow Airport on Saturday, and remanded in custody ahead of a court hearing on Thursday.
He is accused of ordering massacres while head of military intelligence in the wake of the 1994 Rwanda genocide.
Louise Mushikiwabo, Rwanda's foreign minister called the arrest "an outrage".
Gen Karake is also accused of ordering the killing of three Spanish nationals working for Medicos del Mundo.
In 2008, a Spanish judge indicted him for alleged war crimes along with 39 other current or former high-ranking Rwandan military officials.
He was arrested by Metropolitan police officers under the European Arrest Warrant.
But the Rwandan government is said to be puzzled by the timing of Gen Karake's arrest, as he had travelled to the UK several times since the indictment was issued.
Williams Nkurunziza, Rwanda's High Commissioner to the UK, called the arrest "an insult".
"We take strong exception to the suggestion that he's being arrested on war crimes," he told the BBC World Service.
"Any suggestion that any of our 40 leaders are guilty of crimes against humanity is an insult to our collective conscience."
Gen Karake is director general of Rwanda's National Intelligence and Security Services and a member of Rwandan Patriotic Front (RPF).
Nicknamed KK, the Rwandan government hail him as one of the people who stopped the genocide. He went on to be deputy commander of the country's first UN peacekeeping mission in Darfur before returning to his role as spy chief.
During the genocide an estimated 800,000 people were killed between April and June 1994 by ethnic Hutu extremists.
Most of the dead were members of the minority Tutsi community and politically moderate Hutus.
The killings ended when the RPF, a Tutsi-led rebel movement that entered Rwanda from Uganda, seized control of the country.
But Human Rights Watch reported that in their drive for military victory and to control the population, the RPF killed thousands, including government troops, members of the militia and some civilians in numerous executions and massacres.
The Rwanda genocide explained in 90 seconds
Phillip Gourevitch, a leading writer on Rwanda, told Newsnight that Gen Karake's arrest was the equivalent of the head of the UK's MI6 or the US's CIA being arrested.
He said the Spanish indictment had been criticised as highly politicised by many, including US diplomats.
In a statement the British High Commission in Rwanda said the arrest was a "legal obligation" carried out on behalf of the Spanish authorities but stressed the UK had a "close relationship" with the Rwandan government.
"We co-operate closely on a growing range of regional and international issues," the statement said.
"Her Majesty's government greatly values the close relationship with Rwanda and is committed to that relationship for the long term."
Andrew Mitchell, former secretary of state for international development, told the BBC he thought the arrest was "completely wrong".
"I think it is reprehensible that the European Arrest Warrant is being abused in this way," he said.
"It's being used for political reasons and not judicial ones."
He added that he agreed with the American view that the indictments were "un-researched, politically motivated and lacking in facts."