Tens of thousands marched in Madrid in the biggest show of support yet for anti-austerity party Podemos, whose surging popularity and policies have drawn comparisons with Greece's new Syriza rulers.
Crowds chanted "yes we can" and "tic tac tic tac", suggesting the clock was ticking for the political elite.
Many waved Greek and Republican flags and banners reading "the change is now".
Podemos ("We Can") was formed just a year ago, but produced a major shock by winning five seats in elections for the European Parliament in May.
It is currently topping opinion polls in the run up to local, regional and national elections this year.
Spain is emerging from a seven-year economic slump as one of the eurozone's fastest growing countries, but the exit from recession has yet to ease the hardship for thousands of households, in a country where nearly one in four of the workforce is out of a job.
Greek Prime Minster Alexis Tsipras promised that five years of austerity, "humiliation and suffering" imposed by international creditors were over after his Syriza party swept to victory in a snap election on 25 January.
