Left-wing candidate Michelle Bachelet appears as the favorite in the presidential run-off in Chile.
She faces Evelyn Matthei, a former minister in the governing centre-right coalition.
Michelle Bachelet, who was president in 2006-10, won the first round last month, but failed to secure an outright majority.
The contenders are the daughters of air force generals, once friends, who found themselves on opposite sides when General Augusto Pinochet seized power in 1973.
Polls across Chile are due to open at 08:00 local time and will close 10 hours later.
A paediatrician by training, Michelle Bachelet, 62, won 47% of the vote in the first round on November 17. Evelyn Matthei secured 25%.
Everything suggests Michelle Bachelet will comfortably win the run-off.
She leads an alliance of her Socialist Party, Christian Democrats and Communists and has campaigned on policies designed to reduce the gap between rich and poor.
Chile is one of the richest countries in Latin America, but millions have staged protests over the past few years to push for a wider distribution of wealth and better education.
Michelle Bachelet wants to increase taxes to offer free university education and reform political and economic structures dating from the dictatorship of Gen. Augusto Pinochet, who ruled from 1973 to 1990.
Her manifesto this time is much more radical than before, our correspondent says.
Michelle Bachelet was constitutionally barred from serving a second successive term but was very popular when she left office.
Evelyn Matthei, 60, entered the race after two candidates of the centre-right alliance resigned earlier this year – one for alleged financial irregularities, the other one after struggling with depression.
She has called for a continuation of the policies of outgoing President Sebastian Pinera, asserting that Chileans are “better off” now than when he came to power four years ago.
As children in the 1950s, the current rivals were neighbors and used to play together on the airbase where their fathers worked.
Evelyn Matthei’s father, Fernando, rose through the ranks to run a military school.
Michelle Bachelet’s father, Alberto, who was given a job in the Socialist administration overthrown by General Pinochet, died of a heart attack in 1974.
An investigation concluded that the 51-year-old general probably died of heart problems aggravated by torture at the military academy.
A judge ruled earlier this year that General Fernando Matthei had no knowledge of or involvement in the torture.
The Chilean lower house of congress and half the senate are also being elected.
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