PODEMOS has vowed to defeat the PP after it made dramatic gains in the regional elections.
Combined with huge gains for fellow new party Ciudadanos, who also campaigned on an anti-corruption platform, it is clear Spaniards have turned their backs on the traditional two party system.
The governing PP party received the most votes overall but still lost all of its regional majorities.
They have alternated with the PSOE in government for nearly four decades, and combined they received 52% of the vote nationwide, down from 65% four years ago.
“They have both had one of the worst results in their history,” said Iglesias. “This spring of change is irreversible, we will take up the challenge of winning the (parliamentary) elections against the PP.”
Two new political groups inspired by the indignados protests made huge impacts in Madrid and Barcelona.
Ahora Madrid, led by 71-year-old retired judge and grandmother Manuela Carmena, came second behind the PP but could govern the capital if she joins forces with the PSOE, who came third.
While Barcelona en Comun, supported by Podemos, won the most votes in Barcelona. Its leader, anti-eviction activist Ada Colau, 41, now faces the tough battle to pact with smaller rival parties after winning by just one seat.