The Colorado Party's Blanca Ovelar faces a serious challenge
The people of Paraguay have voted in elections that could end 61 years of domination by one party.
The Colorado Party has been in power since 1947, the longest-serving party in continuous rule in the world.
Its candidate, Blanca Ovelar, is facing a serious challenge from a left-wing former bishop, Fernando Lugo, and a retired general, Lino Oviedo.
The opposition has voiced fears of electoral fraud. The Organisation of American States has sent observers.
The Colorado Party's main challenger is Mr Lugo, who brought together leftist unions, indigenous people and poor farmers into a coalition to form the centre-left Patriotic Alliance for Change.
He rejects accusations from outgoing President Nicanor Duarte that he would follow the style of leftist leaders in Bolivia and Venezuela if elected.
Mr Duarte warned that what he called agitators from Venezuela and Ecuador were trying to meddle in the poll.
"My hope is that there are not going to be any clashes and there are not going to be any episodes that might compromise the peaceful living among people and the future of the nation," he said.
The Colorado Party has for the first time chosen a woman candidate, Blanca Ovelar, who has promised to bring a woman's sensibility to a "clearly macho country".
The BBC's Gary Duffy in Paraguay says the most controversial candidate is the former general Oviedo - who was recently released from jail after a sentence for attempting to stage a military coup was overturned.
Poverty remains widespread in Paraguay, particularly in the rural areas.
The economy has been driven by huge hydroelectric schemes and the world's increasing demand for soya.