Kigali: Rwanda said it was an "outrage" for Britain to arrest its intelligence chief at the request of Spain, which wants him on war crimes charges, and suggested Western states were swayed by those behind the 1994 genocide.

Karenzi Karake, 54, director general of Rwanda's National Intelligence and Security Services, was arrested at London Heathrow Airport on Saturday, British police said. He was remanded in custody to re-appear at the court on Thursday.

"Western solidarity in demeaning Africans is unacceptable!! It is an outrage to arrest Rwandan official based on pro-genocidaires lunacy!" Foreign Minister Louise Mushikiwabo wrote on her Twitter account.

The case will likely further strain ties between Rwanda and aid-donor Britain after Kigali suspended a local BBC radio service last year after a documentary by the British broadcaster questioned official accounts of the genocide.

Rwanda has long accused the West and others of doing too little to halt the genocide and then failing to do more to crush groups such as the FDLR "genocidaires", a Hutu militia implicated in Rwanda's 1994 genocide and blamed by the United Nations and human rights groups for atrocities in eastern Congo.

About 800,000 people were butchered in three months of ethnic killings in 1994, most of them from Rwanda's Tutsi minority as well as moderates from the Hutu majority.

The massacre was halted by the Rwandan Patriotic Front, the then rebel force led by Paul Kagame, who is now president. Western states and the United Nations have said they did not do enough to stop the bloodshed and have since poured in aid.

Britain's embassy in Kigali said the arrest "was a legal obligation, following the issue of a valid European arrest warrant", adding that it "greatly values the close relationship with Rwanda" and was committed to it for the long term.

The Rwandan foreign minister accused Spanish non-governmental organizations (NGOs), which she said were behind the arrest warrant, of backing the FDLR.

"The U.N. in 2009 amply documented support of Spanish NGOs behind the preposterous 'valid European arrest warrant' to genocidal militia FDLR!" she wrote in another Twitter note.

Justice Minister Johnston Busingye said Rwanda was working with the British government on the case.

"We will contest in the courts. We have sought explanation from the UK on this matter as well," he said in remarks published by the New Times newspaper on its website.

Rwanda's ambassador in London, William Nkurunziza, told the BBC the charges against Karake were "politically motivated".

In 2008, a Spanish High Court judge, Fernando Andreu, accused 40 Rwandan military and political leaders, including Karake, of engaging in reprisal killings after the genocide.

The judge indicted the officials for genocide, crimes against humanity and terrorism that resulted in the deaths of hundreds of thousands of civilians, including Spaniards.