Exit polls indicate that Peronist hopeful Alberto Fernández has won Argentina's presidential election in the first round, defeating President Mauricio Macri by enough of a margin to win in the first round.
The Juntos por Cambio leader is in second place, with Consenso Federal leader Roberto Lavagna in third, the polls show.
Elsewhere, former economy minister Axel Kicillof is expected to win control of Buenos Aires Province for Frente de Todos, with Juntos por el Cambio's Horacio Rodríguez Laretta set to take first place in the race to be mayor of Buenos Aires City.
All signs are pointing to a defeat for Macri, who has overseen Argentina's descent into economic crisis and recession, with the first initial results due at around 9pm.
Results of voter surveys put Fernández, whose running-mate is ex-president Cristina Fernández de Kirchner, in the lead shortly after polling closed at 6pm local time by a substantial margin.
Local TV news channels just after 6pm began predicting soon after via banner headlines that the president had lost, while other outlets quickly followed suit.
Two exit polls circulated among journalists showed the 60-year-old former Cabinet chief winning by an outright majority, while sources inside the government told at least one local outlet that the government was preparing a concession speech in the expectation that the president would lose in the first round.
Another poll showed that Kicillof had performed strongly against Buenos Aires Province Governor María Eugenia Vidal,
Minutes after the closure of the vote, Frente de Todos campaign official Santiago Cafiero told the press at the party's bunker in Chacarita that the opposition had "improved the electoral performance" it had won in the PASO primaries.
"We are improving on our primary electoral performance both nationally and provincially," said Cafiero, who is close to Alberto Fernández and oversaw his campaign. "We are very confident."
The Interior Ministry said that turnout in Sunday's general election was over 80 percent after a campaign dominated by the crippling economic crisis affecting Latin America's second-biggest economy.
After Argentina slipped into recession last year, the poverty rate has risen to more than 35 percent, inflation for the year to September was at almost 38 percent, while the peso has depreciated 70 percent since January 2018.
Earlier, a relaxed Fernández patiently waited in line to cast his ballot behind other voters, some of whom turned to embrace him. After casting his vote, he vowed to end sharp divisions between his Peronist movement and supporters of Macri.
"The days of 'Us' and 'Them' are over," said Fernández, the Frente de Todos leader, after voting in the Puerto Madera neighbourhood of Buenos Aires City.
"We are in an enormous crisis. Everyone has to take responsibility for what's ahead."
Macri, who brought a bag of medialunas to staff at his polling station in the Palermo district, called on voters for a massive turnout. The president has campaigned relentlessly in the final weeks, visiting 30 locations in 30 days through his '#SíSePuede' tour
Competing "visions of the future are at stake," the president said he cast his vote, admitting that he was "anxiously waiting" for the result.
The election comes amid high tensions in the region, with massive protests in neighbouring Chile and Bolivia, as well as recent unrest over inequality in Ecuador.
Voter María Marta Rosauer, 54, said she would give Macri "another vote of confidence."
"I voted with the conviction and the certainty that he did things well and that he could have done better, but he needed time," she said.
"Nobody can put a country on its feet in four years, after how he found it. We opened our doors to the world after many years of being almost forgotten," she said, referring to the years when Argentina was a market pariah following a 2001 default.
There are "two models of government at stake here. Alberto and Cristina represent greater equity," said another voter, Liliana, a 64-year-old architect in the capital. "I'm excited to see the end of a country that only benefits a small group."
The likely return to power of protectionist Peronists comes amid a lengthy recession and a debt crunch, raising market fears of a possible default on a US$57-billion IMF loan.
The peso fell 5.86 percent in the week before the elections, and the week ended with the dollar at 65 pesos.
Fernández has insisted his government would not default but rather seek to renegotiate the terms of the loan, and sought to reassure voters that their bank deposits would be safe under his administration.
Since Fernández's crushing victory in August primaries, which made him the favorite for the presidency, Argentine savers have withdrawn around US$12 billion from their accounts.
Under Argentine law, a candidate can win the presidency in the first round if they reach 45 percent of the vote, and a 10 point margin over their nearest rival.
Otherwise, a second round will be held on November 24.
Voters will also elect half of the Chamber of Deputies, a third of the Senate, the governor of Buenos Aires Province and the capital's mayor.