Mujica told local television on Sunday that the presidential race will be decided in a one-on-one runoff vote on November 29.
"We're going to fight for the whole nation," he said, "so that the economy works, and also provides for the people who have the least."
The exit polls by three separate companies also showed that the two referendums voted on – one allowing postal votes by citizens abroad and the other to lift an amnesty for human rights violators under the 1973-85 – were too close to call.
Lacalle, the conservative former leader from the National Party, has strong ideological differences with Mujica, who represents the ruling Broad Front and hopes to create enduring socialism in Uruguay.
|If coinfirmed the second round vote will take place on November 29 [AFP]
Lacalle believes in a free market and wants to cut government taxes and distance Uruguay from Latin American leftists. He pushed hard to privatise government as president in 1990-95.
He now wants to remove the income tax imposed by Tabare Vazquez, the outgoing president, and downsize the government.
Vazquez is ending his five-year term on a wave of popularity but is constitutionally barred from re-election.
Mujica's Tupamaru guerrillas were inspired by the Cuban revolution to organise kidnappings, bombings, robberies and other attacks on the conservative but democratically-elected governments of the 1960s.
He was convicted of killing a policeman in 1971, and endured torture and solitary confinement during nearly 15 years in prison.
In the quarter-century since he was freed, Mujica helped transform the guerrillas into a legitimate political movement and the driving force within the Broad Front coalition.
He eventually became the top vote-getter in congress and served as Vazquez's agriculture minister, developing a reputation for populist policies and impolitic commentary.
But there are concerns that the blunt-talking senator lacks the qualities to be the country's president.
"He's a man who simply can't be president of Uruguay… he lacks presence. He can't speak," said Walter Perez, a Colorado Party worker. "If he travels abroad and talks like he does, they're going to throw him out."
Analysts say that if Mujica wins he will likely continue the policies of Vazquez, including plans to cut unemployment, strengthen minority rights and introduce laws allowing homosexuals to form civil unions and adopt children.