Story highlights New party, Podemos, aims to emulate anti-austerity party's victory in Greece

El Pais: About 100,000 people attend rally at Puerta del Sol square

Party's first electoral test at home comes in May in regional and local elections

(CNN) Tens of thousands of supporters of Spain's new anti-austerity party filled central Madrid on Saturday, just a week after a like-minded party won national elections in Greece.

The new Spanish party, Podemos, aims for a repeat of the Greek scenario in Spain, with some polls already show it ahead of the incumbent Conservatives and the main opposition Socialists for national elections due later this year.

Podemos, which means "we can," held its "March for Change" at midday, filling the Spanish capital's emblematic Puerta del Sol square and adjacent streets. It's the same place where anti-austerity protests began four years ago during a deep economic crisis, and Podemos is seen partly as an outgrowth of that movement, which leveraged social media to further its message.

A leading newspaper, El Pais, reported that Spanish authorities estimated about 100,000 people took part on Saturday, while Podemos put the figure at 300,000. El Pais' own estimate, based on people density along the oft-used protest route, was about 150,000.

Whatever the number, it was considered a strong opening shot by Pablo Iglesias, the leader of Podemos, which formed only a year ago. He said Saturday's protest would start the countdown on Spain's conservative Prime Minister Mariano Rajoy, whose party has labeled Podemos as a supposedly dangerous choice that can't possibly deliver on its promises.

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