Lino Oviedo, a former army chief, has also mounted a strong challenge to Ovelar and Lugo.
Polling booths are due to open at 7am (1100 GMT).
The election campaign took a controversial turn as it drew to a close, with Duarte suggesting that Lugo had contacts with oil-rich Venezuela and its left-wing allies.
International election monitors said they have found no evidence of outside intervention in the election.
Lugo says his coalition has appeal for Paraguyans from across the political spectrum.
"I am not of the left, nor of the right. I'm in the middle, a candidate sought by many," said as campaigning drew to a close.
The Patriotic Front for Change consists of about 20 Indian, peasant and union organisations – from left-wing groups to the conservative Radical Authentic Party, Paraguay's largest opposition group.
"The world 'leftist' is used a lot lately in Latin America, and possibly in a mistaken way," Lugo said.
"I believe in the people's self-determination and in recovering sovereignty and independence."
Paraguay has gradually implemented democratic policies since Alfredo Stroessner, an anti-communist general, was removed from power in 1989.
And Lugo has promised to implement policies designed to improve the lives of ordinary Paraguayans, to the concern of big landowners.
Less than two per cent of the population owns more than 90 per cent of the land, and about 40 per cent of people live in poverty.
"We have 300,000 families without land and they have the constitutional right to own the soil they live on," he has said.
Electricity price pledge
Meanwhile, Ovelar says she can steer the ruling party in a new direction, to the benefit of all in the country.
"There are new needs that were not obvious before in our authoritarian state ... and they require the touch of a woman," she says.
Oviedo's candidacy for the presidency comes after a court ordered he be released from jail, citing political persecution.
He has spent several spells in military prisons since he helped overthrow Stroessner 19 years ago.
Lugo and Ovelar have pledged to substantially raise the price of electricity that is sold to Argentina and Brazil - the surplus from Paraguay's share of Itaipu, the world's largest hydroelectric dam.
Alfredo Boccia, a political analyst, told Al Jazeera that Lugo's strong challenge to the Colorado Party could steer Paraguay into new territory.
"There is going to be a period of predictable chaos, especially if Lugo - who has one foot in the conservative right and the other in the left - wins," he said.
"But this chaos opens the way at least for the change that we need, instead of the chaos we already know."