Podemos used its "March for Change" to assail government austerity measures, and paint a portrait of Spain in starkly contrasting hues of "privileged" versus "humiliated" social camps.
In doing so, Podemos amped up the populist themes that have won it considerable support in its mere year of existence - and without detailing many concrete proposals to right the wrongs it denounces.
"Pablo Iglesias and his Podemos party have without doubt scored a victory with the Puerta del Sol rally," conceded edition of Spanish centre-right newspaper El Mundo, which has openly opposed the populist left-wing group since its founding in March 2014.
The event also served as an unofficial kick-off of a busy election year in Spain, starting with regional polling in Andalusia, Catalonia and elsewhere from March-November, municipal balloting in May, and legislative voting in November - when the conservative government of Mariano Rajoy will seek to retain power.
Given the results of recent opinion polls, its understandable why Podemos was first out of the gates to rally backers.
Surveys show Spain's youngest party leading the Socialist (PSOE), and in some polls bettering Rajoy's ruling Progressive Party (PP).
In carefully preparing show of force, Podemos worked social networks to mobilise maximum participation by supporters from all over Spain, who converged on Madrid's Puerta del Sol plaza -- quickly filling it to capacity.
As hoped for by organisers, the photo of that large crowd ran on front pages and as lead stories on media around the world covering the massive turnout.
"We will win elections against the PP," in 2015 vowed Iglesias, 36, sending ripples of excitement through the tightly-packed crowd.
Prior to that Inigo Errejon, 31, and Juan-Carlos Monedero, 52 - number two and three officials in Podemos respectively - described a modern Spain in which two contrasting camps co-existed with increasingly difficulty.
They spoke of a Spain "on top" versus one "on the bottom," of "the powerful" opposing "workers," and of a "privileged" caste lording over the descendants of Spaniards who toiled "to build a country where no one would be cold."
And those people, Iglesias echoed, now suffer "humiliation and increasing poverty," and want to take their fate back into their hands.
"We need courageous dreamers to defend those on the bottom. We need more Don Quixotes," urged Iglesias, evoking Cervantes' famous character.
Podemos rose from the "Indignados" movement born in the streets of Madrid in May 2011 with denunciations of financial markets and political corruption it blamed for the enduring hardship of average Spaniards.
Four years on -- and despite Spain's economy beginning to grow after six years of brutal recession - joblessness still stands at a crushing 23 percent.
In organising rally, Podemos was careful to note its purpose was neither to demand or propose concrete action, but instead mobilise backers for looming elections.
"Speeches were not political, they were emotional messages from the heart to people already dedicated to the cause," noted Catalan paper El Periodico .
Ignacio Urquizu, a sociology professor at the Complutense University of Madrid that Iglesias hails from, drew a similar conclusion.
Though Urquizu says that criticism of Indignados for failing to propose any remedial policies "was unfair, because it was a citizens' protest movement," the same critique of Podemos is valid because it "seeks power in our country while demonstrating the same failing," he wrote in El Pais.
Podemos' vague programme is inspired by the socialism of late Venezuelan president Hugo Chavez, and progressive Scandinavian progressive parties. The movement even refuses to be labelled leftist.
it continued airing a list of rather generic pledges that included battling corruption, reversing the widening wealth gap, and defying the so-called "troika" imposing austerity on indebted European states by demanding the restructuring of Spanish debt -- which currently represents 96.8 percent of GDP.
In doing so, Podemos says it is responding to the hopes of many Spanish voters who no longer believe in the PSOE and PP that have alternated stints in power for more than three decades.
"Voters don't read 400 page party programmes," noted political analyst Jose Ignacio Torreblanca, adding the PSOE itself rose to power in the 1980s using similar inspirational methods - and nearly the exact "For Change" slogan that Podemos marched under .