A strong showing by the radical left threw Chile's presidential race wide open and left billionaire Sebastian Pinera with a fight on his hands to secure victory in a runoff, analysts said Monday.

Center-right ex-president Pinera emerged from the first round in front as expected, albeit with a much lower than anticipated 36.6 percent, as Chileans went to the polls on Sunday to choose a successor to Socialist President Michelle Bachelet.

Pinera, 67, faces Bachelet's candidate Alejandro Guillier, a 64-year-old former TV news anchor, in the second round on December 17.

But the big winner was new radical left party Frente Amplio, and its leader Beatriz Sanchez, who is set to play a key role in the second round even though Guillier pipped her to the runoff spot by just 160,000 votes.

Sanchez's emergence is "the biggest shock that Chilean politics has had since the return of democracy in 1990," analyst Mauricio Morales from the University of Talca told AFP.

"While polls had pointed to her gaining 13 percent of the vote, her strong outturn of 20.3 percent could mean that a larger than expected number of votes go to Mr. Guillier in the second round," Analysts Capital Economics said in a note.

Santiago Stock Exchange's IPSA index was down 4.5 percent shortly after opening Monday on news of market-friendly Pinera's weaker than expected showing.

The billionaire's Chile Vamos coalition performed weakly in the parallel parliamentary elections, falling short of a majority, which analysts said could hinder his economic agenda.

Pinera's promised market-friendly reforms looked less likely, particularly his plans to lower the corporate tax rate -- which Bachelet has just raised -- and cut red tape in the mining sector, Capital Economics said.

- Faulty polling -

Opinion polls taken before Sanchez's strong showing showed Pinera would comfortably win a runoff against Guillier, who polled 22.7 percent of the vote.

While Pinera has a big lead, he had been expected to poll up to 43 percent in the first round.

Analysts said Pinera will now be forced to appeal to the far right for support in the second round, after extreme right-candidate Jose Antonio Kast polled strongly, taking 7.9 percent of the votes.

"Pinera came out with eight or nine points below what was expected and this is an ingredient that provides drama and uncertainty to the second round," analyst Marcelo Mella of the University of Santiago told AFP.

"The key will be what the contingent who voted for Beatriz Sanchez will do."

Right wing Senator Andres Allamand said Sanchez's emergence had changed the landscape of Chilean politics, adding a third force to the traditional coalitions of the left and right. "It's a political event of great magnitude," he said.

Sanchez, 46, a seasoned journalist who entered politics only in March, polled more than 1.3 million votes as a representative of the anti-austerity Frente Amplio party. The party also boosted its representation in congress, surging from three to 20 seats.

"The left-wing populist Frente Amplio has enough seats to be a nuisance in Congress. This could ultimately mean that some of Mr. Pinera's reforms are diluted," said Capital Economics.

Mirroring the rise of similar radical parties in Spain and Greece, the new party has drawn together leftist movements, student leaders and disenchanted former Bachelet supporters.

She rounded on the pollsters in a speech to supporters late Sunday, saying they had failed to provide a true picture of her party's public support.

Jointly, the six leftist candidates in the presidential race polled 3.6 million votes between them, while the two right-wing candidates took just 2.9 million votes.

Source:AFP