Public opinion poll results released Friday put Mr. Mujica ahead of Mr. Lacalle by about eight percentage points
Voters in Uruguay are casting ballots in a run-off presidential election that pits a former guerrilla leader against a former president.
Public opinion polls indicate Jose Mujica, a former leftist rebel who spent 14 years in prison, is poised to become the country's next president.
He faces Luis Alberto Lacalle, who led the country from 1990 to 1995.
Uruguay organized Sunday's run-off vote after Mr. Mujica fell just short of winning an absolute majority in the October election.
The winner will replace President Tabare Vazquez, who is barred from seeking a second consecutive term.
Both candidates have pledged to maintain the fiscal policies of the popular Vazquez administration, which have helped Uruguay's economy expand despite the global recession.
Preliminary results are expected an hour after polls close Sunday (at 2130 UTC).
In the 1960s, Mr. Mujica was part of the Tupamaro guerrilla movement that used bank robberies, political killings and kidnappings to fight Uruguay's conservative government.
He was imprisoned during most of Uruguay's military dictatorship from 1973 to 1985.
Some information for this report provided by AFP, AP and Reuters.