Supporters of Senator Jose Mujica celebrate after the polls close
A former left-wing guerrilla has taken a commanding lead in several exit polls from Uruguay's presidential election.
Most polls show Senator Jose Mujica has narrowly failed to secure the 50% needed to avoid a run-off vote.
If a second round is officially confirmed, he is likely to face his main conservative rival, the former president, Luis Alberto Lacalle.
The winner will replace outgoing socialist President Tabare Vazquez and take office in March next year.
Exit polls on Sunday suggested 74-year-old Mr Mujica had gained about 48% of votes, with Mr Lacalle trailing on around 30%.
A 29 November run-off would take place between the two frontrunners.
Mr Mujica, a senator of the governing left-wing Broad Front Party, was a former member of the rebel Tupamaros movement in the 1960s and 1970s.
Mr Lacalle, famed for having survived an attempt to poison him - and other National Party leaders - with tainted wine in the 1970s, is a 68-year-old lawyer.
He has pledged to remove the income tax imposed by President Vazquez and trim the size of government in the country of 3.4 million.
During the election campaign, he sought to capitalise on concerns among some voters about his rival's militant past.
Mr Lacalle has crafted a political comeback 14 years after leaving office when his senior aides were accused of corruption.
As well as presidential and congressional elections, voters also took part in a referendum.
That will decide whether to revoke a law which gives immunity to former security officials accused of human rights abuses during Uruguay's period of military rule.