Among those chasing qualification will be Brazil, winners of the Olympic gold medal for the first time in the country's history at Rio 2016. 

Like every other team in the tournament, though, Brazil will be without some leading players because the tournament, organised by the South American Football Confederation (CONMEBOL), is not an official FIFA event, so clubs have no obligation to make players available.

Brazil have had to replace five players originally named in its squad, including Real Madrid's Rodrygo and Vinicius Júnior and Arsenal forward Gabriel Martinelli.

"As coach, what I can do is to call them and then it is up for the clubs to collaborate," Brazil's coach Fernando Batista told Associated Press.

Argentina, the 2008 Olympic champions with a squad that included Lionel Messi, Pablo Zabaleta, Ángel Di María, Javier Mascherano and Sergio Agüero, have been similarly hit by unavailabilties.

They have been denied the services of Inter Milan's Lautaro Martínez and Lisandro Martínez of Ajax. 

But Brazil are still able to call on the services of Reinier Jesus, already a regular first-teamer with Flamengo who is expected to join Real Madrid in the summer.

Argentina, meanwhile, have Adolfo Gaich, who, at the age of 20, has played for the senior team. 

It is the first time since 2004 that the Pre-Olympic Tournament has been staged. 

For the last three Olympic Games, the South American Youth Championship was used as the qualifying event.

Colombia are in Group A with Argentina, Chile, Venezuela and Ecuador, while Brazil feature in Group B with Uruguay, Peru, Paraguay and Bolivia.

The top two in each group advance to the second round, with the winner and runner-up from the final on February 9 qualifying for Tokyo 2020.

History suggests that Brazil and Argentina will be favoured.

In the 12 previous editions of the tournament, Brazil have won it seven times and Argentina four, with only Paraguay stopping South America's superpowers when they won in 1992. 

There will be special incentive for Bolivia and Ecuador, as they are the only two CONMEBOL teams yet to take part in a men's Olympic football tournament.

Ecuador are set to be the first country in action, when they meet Chile in the opening match of the tournament at Estadio Hernán Ramírez Villegas in Pereira.

Hosts Japan are already qualified for Tokyo 2020, along with France, Germany, Romania and Spain from Europe, New Zealand from Oceania and Egypt, Ivory Coast and South Africa from Africa.