MADRID — At least 100,000 people marched through Madrid on Saturday in a show of strength by a fledgling radical leftist party, which hopes to emulate the success of Greece’s Syriza party in the Spanish general election this year. Podemos seeks to shatter the country’s predominantly two-party system, and the March for Change gathered crowds in the same place where sit-in protests against political and financial corruption laid the party’s foundations in 2011. The party’s rise is largely because of the charisma of its leader, Pablo Iglesias, a 36-year-old political science professor. Opinion polls show the party could take the No.