Billionaire conservative Sebastian Pinera, who held a commanding lead in Chile's first round of presidential voting, and leftist former TV journalist Alejandro Guillier will contest a runoff next month.
Ex-president Pinera had a 36.6 percent lead to Guillier's 22.6 percent with more than 90 percent of the votes counted after the first round.
"Tonight we have achieved a great electoral result and above all because we have opened the doors which will lead us to better times," Pinera told his triumphant supporters.
Guillier, an independent supported by President Michelle Bachelet's Socialist party, beat off a stiff challenge from an unheralded far-left candidate, Beatriz Sanchez, for the second runoff place.
Pinera, a 67-year-old billionaire who was president from 2010 to 2014, had been the clear favorite going into Sunday's first round. The second round is set for December 17.
"The result is very similar to the one we had in 2009, and in 2009 we won the election, and we managed to get our country up and running," said Pinera, who campaigned on reviving an economy that has suffered years of weak growth.
Guillier, a 64-year-old news anchor turned Senator, said Chile "wants another way, and has expressed that in the vote," which gives him a platform to build on in the second round if he can unify a fragmented left.
"The result completely reconfigures the Chilean political landscape," University of Santiago analyst Rene Jara told AFP after both far-left and far-right candidates polled more strongly than expected among the eight presidential candidates.
Sanchez, 46, a seasoned journalist who entered politics only in March, polled more than 1.2 million votes as a representative of the anti-austerity Frente Amplio party.
"Chile wants change and said it today by voting," she said.
Jara said Sanchez's vote had given her party "very strong negotiating power for the second round."
Although her party had been reluctant to pledge support for Guillier, "they are obliged to do so because they will not be responsible for a return of Pinera to power."
Analysts said Pinera will 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.
- Voter apathy -
Bachelet, who was also Chile's first woman president, hugged and took photographs with female supporters before casting her ballot in Santiago.
"It is important that people come out and vote (for a candidate) because they feel they represent what they want for Chile," she said, predicting a second round.
As he headed to a Santiago polling station, Fernando Aravena, 76, said: "We need change. That's the idea."
Daniel Concha, a 31-year-old psychologist, said it was "very likely" that Pinera would win the election.
Paula Salas, 35, said she voted for Sanchez.
"If she became president, she would do more, because she has fewer links with the powerful class," Salas said.
Lawyer Cristian Barros cautioned against a Pinera victory being seen as a foregone conclusion.
"There were several former presidential candidates in Chilean history who were considered winners, and I think Pinera should not be given as a winner until the end of the election," said Barros, 37.
Chile's constitution bans consecutive terms for presidents, but re-election after skipping a term is permissible.
Bachelet herself led the conservative South American country -- Latin America's fifth-largest economy -- from 2006 to 2010 and then was re-elected to replace Pinera in 2014.
Compulsory voting was dropped in 2012. Since then, a growing number of Chile's 14 million eligible voters have decided to stay away from voting booths.
- No majority? -
Pinera's first presidential victory in 2009 elections signified a break from the center-left politics that had reigned in Chile since democracy was restored with the end of Augusto Pinochet's dictatorship in 1990.
But a Pinera comeback is not seen as a rejection of the overall economic and social model erected in the Bachelet years, during which Chile posted annual growth of 1.8 percent and passed tax and labor reforms, as well as introduced free education and the right to abortion.
"Chileans don't want to tear down the model, just fix its structure," said political analyst Mauricio Morales of Talca University.
Pinera has promised modifications to Bachelet's reforms, and vowed to have Chile join the club of developed nations within eight years.
His effectiveness, though, could be hobbled by a shortfall in legislative support.
"He is not going to have a majority in Congress," predicted analyst Marta Lagos, founder of Latinobarometro and MORI Chile.
Sunday's elections also included legislative elections for 155 congressional seats and half the Senate.
The right was set to increase its share of the seats, but not by enough to have the majority in either chamber. It is the first poll in the country's history that includes expatriate citizens.