Chanting "No to war" and calling Spain's progressive government "imperialist," the protesters, mainly from the far-left of the political spectrum, marched through Paseo del Prado boulevard.
Spain's Defense Minister Margarita Robles, however, was quick to point out that the anti-NATO activists were a "clear minority" in Spain, speaking in an interview with news agency EFE on Sunday, Anadolu news agency reported.
This week, the Spanish capital will be dominated by the military alliance, as NATO leaders are set to meet in Madrid to discuss what could be its largest military deployment since the end of the Cold War.
The NATO Summit will run from Tuesday to Thursday, with talks expected to center on NATO's response to the war in Ukraine and the Finnish and Swedish petitions to join the alliance.
Robles said that Spain, as a close ally of Türkiye, is offering to act as an intermediary to reach an agreement around the two Nordic countries joining NATO.
MA/PR