Former guerrilla fighter Jose Mujica led Uruguay's presidential runoff election, a quick count projection by the Factum polling group showed.
Factum said Mujica was leading with 51.2% over centre-right former President Luis Lacalle, based on its projections.
The vote is seen as a referendum on the economic success of the country's ruling leftist coalition.
Mr Mujica, 74, who waged an armed revolt against Uruguay's democratically elected government in the 1960s and 1970s, leads by at least six points in opinion polls over his center-right rival, former President Luis Lacalle.
A Mujica victory would keep in power the ruling Broad Front coalition credited by many Uruguayans with lifting the country out of an economic slump earlier this decade and stoking growth this year in the face of the global slowdown.
A farmer and former agriculture minister and senator, Mr Mujica has vowed to continue the investor-friendly policies that have helped the economy expand for six straight years.
He won the most votes in the election's first round on 25 October, finishing with 48% to 29% for Mr Lacalle, and falling just short of the outright majority needed to avoid a second round. The winner in the run-off assumes office on 1 March for a five-year term.
Former President Pedro Bordaberry, who finished third in the initial round, has thrown his support behind Mr Lacalle.
Mr Lacalle has raised questions about Mr Mujica's militant past, suggesting his opponent would be a more radical leader than he has portrayed himself in the campaign. But Mr Mujica has pledged to maintain the moderate policies of current President Tabare Vazquez, Uruguay's first socialist leader.
Mr Lacalle, a 68-year-old lawyer from the National Party who governed from 1990 to 1995, has said he too would leave economic policy intact. He has pledged to fight crime, lower income taxes and reduce the size of government.
Mr Mujica was once a member of the Tupamaros Marxist guerrilla movement that sought to weaken Uruguay's conservative government during the turbulent 1960s and 1970s through robberies, political kidnappings and bombings.
He said he was shot six times and at one point held by security forces in solitary confinement in a deep well. He was imprisoned during most of Uruguay's 1973-85 military dictatorship.
The Tupamaros later transformed into a political party and joined with socialists and other leftist parties to create the Broad Front coalition, which took power in 2005 during South America's political shift to the left.
Mr Mujica, who is popular with the Uruguayan poor and working class, is lauded by many of his supporters and eyed with caution by critics for being outspoken and blunt.
