Marta Lagos, director of the polling group Latinobarómetro, said low voter turnout that could be explained in large part by a general view that politicians are corrupt and public institutions are not to be trusted, she said.
Four members of Congress have been stripped of immunity from prosecution and dozens of politicians, their associations and relatives are being investigated for illegal campaign financing in earlier elections, influence-peddling and other white-collar crimes.
“People are disappointed with our democracy,” Ms. Lagos said. “Parties and politicians have been totally discredited, and there is a widespread perception that all politicians are corrupt demons, that there is no one to vote for.
“People want a major transformation of the political system and corrupt politicians put in jail, and maybe then they will get out and vote.”
On Sunday morning, a group of demonstrators forced their way into Mr. Piñera’s campaign center to protest the elections. More than 20 were arrested. In the Araucanía region, about 400 miles from Santiago, the capital, unidentified individuals intercepted a bus that was going to transport voters from rural areas to voting places and set it on fire.
Both the right-wing opposition and the governing coalition, New Majority, have two candidates competing against each other for the presidency. Mr. Piñera is flanked to the right by José Antonio Kast, an ultrarightist who proposes actions like building a physical barrier along Chile’s northern border with Peru and Bolivia, withdrawing from international treaties and organizations, and abolishing a recently approved law permitting abortions in some cases.
Against all predictions, Mr. Kast was so far drawing about eight percent of the vote.
Mr. Guillier was competing with a political ally, Carolina Goic, of the more conservative wing of the centrist Christian Democratic party. The remaining three candidates are all to the political left of Mr. Guillier.
If Mr. Piñera and Mr. Guillier go on to a second round in December, they will have to cater to the electorate of their former competitors to win. The surge from the left, more relevant than in any other presidential elections since 1990, may change the political map, or at least influence the agenda of the future president.
For the first time, political power may not be wielded solely by the two major coalitions that have governed Chile since the end of military rule.
“We’ve been living in a democracy full of constraints, with the same politicians in government and Congress for too many years,” said Patricia Bravo, a 56-year old drug rehabilitation therapist who supports Ms. Sánchez. “They already fulfilled their role, which was to transit towards democracy, but it is time to change. This election is an opportunity to start anew.”
Ms. Bachelet will finish her four year term in March 2018. Under Chile’s Constitution, she cannot run for immediate re-election. Battered by criticism, sagging popular support and a high-profile corruption case involving her son and daughter-in-law, Ms. Bachelet has publicly vowed that she will not seek any elected office in Chile in the future.