The timing of Syriza's victory in Greece's snap vote on January 25th has proved ideal for the radical Spanish party Podemos, which was already planning Saturday's "March for Change" in Madrid's Puerta del Sol square.
"Hope is born," the Spanish party's pony-tailed leader Pablo Iglesias, a former university politics lecturer, said after the Greek result.
Syriza beat the mainstream Greek parties, as Podemos aims to do in Spain's general election due in November.
Iglesias, 36, appeared alongside Syriza's Alexis Tsipras to publicly support him during his campaign.
Podemos was formed just a year ago, but has surged in the opinion polls with promises to fight what Iglesias calls the traditional "caste" of political leaders.
"Tick, tock… it's the final countdown for the ruling caste," Iglesias told supporters on the day of the Greek election.
Podemos has distributed leaflets in the street ahead of Saturday's rally calling for a "politics that serve the people, not private interests".
It hopes to draw tens of thousands of supporters by bus, train and car to the capital.
Demonstrators will meet at Madrid's Cibeles Square at midday before marching to the Puerta del Sol along the broad avenues that have seen many demonstrations against the government's crisis cutbacks.
Protesters known as the Indignants filled that square for weeks in 2011 demanding political change at the height of Spain's economic crisis, and countless more protests followed.
Spaniards were enraged at numerous reports of political corruption, as well as public spending cuts imposed by the conservative ruling party and previously by the Socialists after the economic crisis erupted in 2008.
Spain has now officially exited recession but nearly one in four workers is still unemployed.
Podemos blames the crisis on "a corrupt minority which has committed the biggest plundering in Spain's recent history".
The street protest movement has died down since 2013 but some of the Indignant leaders formed Podemos in January 2014.
Four months later, the party won five seats in the European Parliament, with more than 1.2 million Spaniards voting for it.
Buck the system
Podemos has overtaken the mainstream opposition Socialist Party (PSOE) in several opinion polls, and in some has topped the list ahead of the conservative ruling People's Party (PP).
The Socialists and the PP have ruled Spain alternately since the country returned to democracy after the death of the dictator Francisco Franco in 1975.
By threatening to break up the old two-party system, Podemos has provided the old rivals with a new common adversary, which they brand populist.
Prime Minister Mariano Rajoy has warned Spaniards not to "play Russian roulette" by supporting Podemos, which he said "promises the moon and the sun" but will not deliver.
Media on left and right have gone after Podemos, accusing it of links to Venezuela's left-wing leaders and alleging fiscal irregularities by some of its top members.
The party's leaders have promised to publish their tax returns to dispel the allegations.
Iglesias has responded defiantly to the pressure.
"In the face of their hatred, we smile," runs one of his regular refrains.
Podemos has largely mobilised via online social networks and organised voting to choose its leaders online. Calling a mass rally, as it has for Saturday, is a first for the party.
"It is a risky bet… a demonstration in the streets which could disappoint expectations," said analyst Jose Ignacio Torreblanca, who is writing a book on Podemos.
"But it is true that demonstrating together shows that they are real and not virtual."