The leader of Spain's Podemos party, which came third in Sunday's general election, says he would not allow the ruling People's Party (PP) to form a new government.
"Podemos would not either actively or passively permit a PP government," Pablo Iglesias told a news conference on Sunday.
The ruling conservative party of Prime Minister Mariano Rajoy won the most seats - 123 - at the general election but fell far short of the majority of votes in the 350-seat parliament needed to form a new government.
Podemos won 69 seats while the Socialist Party (PSOE) came second with 90 seats.
The PP and the Socialists have been running the country in turn for more than three decades.
The leader of Podemos, which has shaken up the politics in Spain, says he has no "doubt" that he wants nothing to do with the ruling party.
The Podemos ("We can") party entered the mainstream for the first time in Sunday's parliamentary vote, ending decades of two-party domination and raising the possibility of new elections.
The left-wing party was founded by political scientist Pablo Iglesias in March 2014 in the aftermath of the Indignants' nationwide protests against inequality and corruption which began in May 2011.
The Indignants movement demanded a radical change in Spanish politics, as demonstrators did not consider themselves to be represented by any traditional party nor favored by the measures approved by politicians.
Podemos and the liberal Ciudadanos ("Citizens") parties claim they seek to fill that gap by addressing inequality, corruption and unemployment that followed in the wake of the European debt crisis, which has been going on since the end of 2009 and has affected several eurozone member states including Spain, Greece, Portugal, Ireland and Cyprus that were unable to repay or refinance their government debt.
Podemos has called for a renegotiation of Spain’s international debt and government austerity measures.
Its leader said he would soon begin a round of talks with all other political parties to discuss alternative solutions.
Unemployment still remains high at more than 21 percent in the European state, but the ruling party claims the government’s austerity measures and job reforms have saved Spain from a financial crisis and placed the country back on the track of economic growth.
The Spanish government says it expects the economy to register a 3.3-percent growth this year.