The BBC's Daniel Schweimler says the funeral will be a national and international affair
Thousands have converged on Argentina's government palace to pay their respects to ex-President Nestor Kirchner, who died on Wednesday at 60. His body is lying in state to allow people to file past in honour of the man who was president from 2003 to 2007.
His wife Cristina Fernandez and their children have been attending the wake.
Mr Kirchner, succeeded by his wife as president, was expected to run in the 2011 election.
Relatives, government ministers and politicians have participated in the wake, while a steady stream of Argentines have been filing into the government palace to pay their respects.
Many were carrying candles, flags and flowers. Some applauded, others shouted "Nestor, Nestor" and "Nestor is not dead, he will live on in the people," while walking past the coffin.
President Fernandez, dressed in black and wearing dark glasses, was joined at the wake by South American presidents and friends, including former soccer player Diego Maradona.
She placed a hand on her husband's flag-draped coffin, with their son and daughter standing next to her.
It was the leader's first appearance in public since her Mr Kirchner died of a heart attack Wednesday aged 60.
On Wednesday night, thousands took to the streets of Buenos Aires to voice their grief.
Mr Kirchner's supporters gathered in front of the government palace, the Casa Rosada or Pink House, waving Argentina's blue and white flag, lighting candles and leaving flowers.
Mr Kirchner had suffered health problems and had a heart operation last month but nevertheless his death shocked many in Argentina, where three days of national mourning were declared.
His body was flown back to the capital, Buenos Aires, early on Thursday from the southern town of Calafate, where he died in hospital with his wife by his side.
The presidents of Bolivia and Ecuador, Evo Morales and Rafael Correa, were the first regional leaders to arrive in Buenos Aires to attend the funeral of Mr Kirchner, who was also the first general-secretary the bloc of 12 South American nations Unasur.
"A great patriot has gone, a great Latin American," Mr Correa said.
"The best homage we can pay to Nestor is to reiterate our commitment to build this great country, this united America that he dreamed so much of and fought hard for as the head of Unasur," Mr Correa added.
The Uruguayan president, Jose Mujica and Chilean leader Sebastian Pinera also travelled to Argentina.
Several other Latin American leaders are flying in to attend the funeral.
The Venezuelan President Hugo Chavez arrived in Buenos Aires on Thursday evening and told reporters that "a just and brave man has died".
Brazil's President Luiz Inacio Lula da Silva has cancelled all his appointments in order to go to Argentina and has declared three days of mourning in Brazil.
The Buenos Aires Herald reports that about a thousand people are passing through the palace every hour, in groups of 150-200 people.
Visitors will be allowed in throughout the night and until midday on Friday.
The former president's body will then be flown to the southern city of Rio Gallegos, where he spent much of his political career.
It is understood that the wake will continue there, with the actual burial now scheduled for the weekend.
The country's football matches this weekend have been called off following his death.
All matches from the first division to lower semi-professional leagues were suspended, the Argentine Football Association said.
Return to stability
Mr Kirchner came to power as Argentina was emerging from a profound political and economic crisis, and he oversaw the country's return to relative stability and prosperity.
He also supported the prosecution of those responsible for human rights abuses under military rule in the 1970s and 1980s.
Mr Kirchner was a polarising figure, says the BBC's Daniel Schweimler in Buenos Aires.
He was very popular among the trade unions and in the industrial belt around Buenos Aires, deeply unpopular among the wealthy, especially so among the influential agricultural lobby which did not like him nor his wife.
His death leaves a huge void in Argentine politics, which both his supporters and opponents will now have to face, our correspondent adds.
The couple had faced some criticism for appearing to get around the constitutional limit on two consecutive terms.
Just as Mr Kirchner stood aside for his wife in 2007, it was widely thought President Fernandez would step back and allow him to run in the October 2011 election.
Very powerful images. The people coming together and shouting their words of support, all the South American leaders side by side... It sure feels like the end of an era in Argentina.