By Julien Toyer

MADRID (Reuters) - A party formed in January has become Spain's main political force one year before national elections, an opinion poll showed, shaking up a two-party system that has dominated since the country's return to democracy in the 1970s.

Podemos ("We Can") has ridden anti-establishment sentiment for months by fielding young candidates new to politics and tapping into the "Indignados" movement that occupied Spanish squares three years ago calling for a new political model.

Its support has risen steadily in surveys, adding pressure on the ruling People's Party and opposition Socialists as they battle Catalonia's independence drive, corruption cases in their ranks and a depressed economy.

But Sunday's Metroscopia poll in daily El Pais was the first to mark the newcomer, headed by 36-year-old political science lecturer Pablo Iglesias, as coming first in a national election.

It showed Podemos winning 27.7 percent of the vote if it was held today, just ahead of the Socialists with 26.2 percent and with the PP trailing a distant third with 20.7 percent.

"A political earthquake," said El Pais in an editorial published with the poll.

"The current picture reflects a monumental anger from the people and spells uncertainties for the still far-away general election of 2015... This situation reflects the failure from the main forces of the political system and reveals an impending risk of a breakup."

It also echoes anti-establishment sentiment elsewhere in Europe, as demonstrated by growing support for former outlier parties such as Britain's UKIP and Italy's 5-star Movement.

Although Metroscopia cautioned the poll, which questioned 1,000 people, lacked key historical background to adjust the results of Podemos, it also showed that 47.7 percent of the electorate would not vote or would cast a blank ballot.

If replicated on voting day, it would also rule out the scenario of a grand coalition between the two main parties, who would together secure less than percent of ballots compared with around 80 percent before the economic crisis.

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FURY

Prime Minister Mariano Rajoy's PP won an absolute majority in 2011, which proved an asset during the euro zone debt crisis as it enabled the government to pass unpopular reforms to rebuild the economy and shielded Spain from Italian-style political instability.

Rajoy has pinned his main hopes on winning a second term next year on the economy, expected to grow this year and next after exiting a five-year slump in the second half of last year.

But the recovery is feeding unevenly in Spaniards' daily lives, with the health and education systems still suffering deep spending cuts and unemployment, currently just below 24 percent, expected to stay high for another few years.

A slew of corruption scandals in recent months involving politicians, high-profile company executives and the king of Spain's sister, have also sparked public fury, and forced Rajoy to apologize for the first time for the cases affecting members of his party.

According to the poll, 91 percent of Spaniards believe the political situation in the country is bad or very bad and 81 percent disapprove of Rajoy's actions as Prime Minister.

Podemos, meanwhile is trying to smooth out a policy slate that initially called for Spain's biggest companies to be nationalized its debt defaulted on.

The party, which unexpectedly took five seats in May's European elections, now says it will carry out nationalizations only if the firms don't fulfill their social responsibility and will "audit" the debt before making any further decision.

It is also building up its structure across the country, and Iglesias is set to be confirmed as leader in mid-November after winning strong support in party assemblies.

Telegenic and social media-savvy, Iglesias is the only one among Spain's leading political figures to have a positive rating along with new king Felipe VI and his wife Queen Letizia.

(editing by John Stonestreet)