The first group stages of the women's tournament take place on Wednesday - so-called day -2 - with Sweden facing South Africa, while the men's group stages start on Thursday, when Iraq play Denmark (both 17:00 BST).

Here's all you need to know.

Why does football start early at the Olympics?

It's simply for scheduling reasons, as it was at London 2012.

With 16 men's and 12 women's teams, there are 58 matches to get through. There is football being played on 12 days of the tournament, with six rest days.

Men's groups

Group A: Brazil, South Africa, Iraq, Denmark

Group B: Sweden, Colombia, Nigeria, Japan

Group C: Fiji, South Korea, Mexico, Germany

Group D: Honduras, Algeria, Portugal, Argentina

Women's groups

Group E: Brazil, China, Sweden, South Africa

Group F: Canada, Australia, Zimbabwe, Germany

Group G: USA, New Zealand, France, Columbia

Which stars are in action?

Barcelona forward Neymar will captain a Brazil squad aiming to win its first Olympic gold medal.

The hosts also have Lazio midfielder Felipe Anderson in their squad along with Neymar's Barca team-mate Rafinha and Paris Saint-Germain defender Marquinhos.

Meanwhile, Tottenham forward Son Heung-min will represent South Korea and Chelsea's John Mikel Obi brings experience for Nigeria.

Five-time women's world player of the year Marta competes in her fourth Olympics for Brazil, while her 38-year-old team-mate Formiga is going to her sixth Games and is looking to continue her streak of appearing at every women's Olympic football tournament.

The reigning world and Olympic champions USA have seven gold medal winners in their line-up. Among them is goalkeeper Hope Solo, who could win a record fourth Olympic title at Rio, and current world player of the year Carli Lloyd.

Team GB fielded men's and women's teams at London 2012, with both sides knocked out at the quarter-final stage.

The English Football Association had hoped to have both teams in Rio, but they failed to gain support from the other home nations and Fifa was not willing to accept a Great Britain team without players from every one of the home nations.

Qualification is also based on tournament results and while England's women would have earned their place by finishing third at last year's World Cup, the men's failure to get past the group stage of the European Under-21 Championships would have denied them.

Team GB qualification is based on England's results because the English FA is the national governing body on the BOA.

"I'm really disappointed that these next set of players won't have that opportunity in Rio," said former England striker Kelly Smith, who was part of Team GB's 18-strong squad at London.

"It's a great experience and you get to play against the best teams and players from across the world. It's disheartening they couldn't get a side together. The Americans view this as the ultimate."

How big is Olympic football?

Even though it was not included in the Olympic programme until 1996, the women's competition is often seen as more prestigious than the men's, which is age-restricted (only three men in each squad can be over 23).

For many Brazilians, the men's football tournament will be the most important part of the Games - especially on home soil, where they are looking to win their first ever gold medal in the sport.

Who are the favourites?

Hosts Brazil are the firm favourites to take the men's title after the disappointment of losing in the final at London 2012.

Defending champions Mexico could prove a difficult opponent again, while Germany also have a strong team.

In the women's event, the USA won gold in 2012 and the World Cup last year, while the European trio of France, Germany and Sweden will also be strong.

(BBC)