Some 200 people gathered outside the British High Commission in Rwanda to protest at the arrest of intelligence chief Karenzi Karake in London.
Gen Karake, 54, was detained at Heathrow Airport on Saturday, accused of ordering massacres in the wake of the 1994 Rwanda genocide.
Protestors have threatened not to end the demonstration in the capital, Kigali, until the general is released.
The Rwandan government has branded the arrest an “outrage”.
Louise Mushikiwabo, Rwanda’s Foreign Minister condemned Gen Karake’s detention on Tuesday, saying that “Western solidarity in demeaning Africans is unacceptable”.
“We are here to stand in solidarity with our hero, who was among the people who stopped genocide in this country,” Herbert Muhire, a protest leader told The Associated Press news agency.
William Gelling, the UK’s High Commissioner to Rwanda, addressed the crowd briefly.
“All I can say is that this was a legal decision as you understand, on behalf of the Spanish legal authorities.
“The UK is a very close partner with Rwanda,” he said, as quoted by AP.
Gen Karake was arrested by Metropolitan Police officers in response to a European Arrest Warrant.
Spanish investigative judge Andreu Merelles indicted Gen Karake in 2008 for alleged war crimes, along with 39 other current or former high-ranking Rwandan military officials.
He is also accused of ordering the killing in 1997 of three Spanish nationals working for Medicos del Mundo.
Nicknamed KK, the Rwandan government hail Gen Karake as one of the people who stopped the genocide in 1994.
An estimated 800,000 people were killed in two months 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, seized control of the country.
Gen Karake is director general of Rwanda’s National Intelligence and Security Services and a member of Rwandan Patriotic Front (RPF).
But Human Rights Watch reported that in their drive for military victory, the RPF killed thousands, including government troops, members of the militia and some civilians in numerous executions and massacres.
A spokesman for Spain’s National Court said it was now up to the British authorities to decide whether or not to extradite Gen Karake.