Rio Olympics organisers said they were "not disappointed" by the number of empty seats on show as the athletics events began in the Estadio Olimpico.
They claimed 58% of available tickets for the first session were sold, and 65% for the evening session.
Ticket prices and VIPs not filling their allocations have been cited as reasons for empty seats.
"Without heroes and local athletes the public take longer to fall in love," said Rio 2016 spokesman Mario Andrada.
"But one of the legacies of the Games will be to show Brazilians how spectacular these other sports can be and there will be no problem when Usain Bolt competes on Sunday."
Jamaica's Bolt, 29, is attempting to win the 'treble treble' of 100m, 200m and 4x100m golds at three successive Olympics.
Andrada said an additional problem in Rio is ticket-holders only wanting see one particular contest in a long session.
The athletics began on Friday with a world record for Ethiopia's Almaz Ayana in the women's 10,000m, and Britain's Jessica Ennis-Hill opening her defence of the heptathlon title she won in London.
However, media outnumbered fans at the start of competition, and by lunchtime the stadium was barely a quarter full.
For the start of the evening session, which featured the likes of Jamaica's two-time Olympic 100m champion Shelly-Ann Fraser-Pryce, the stadium was less than a fifth full.
Rio 2016's organising committee has been criticised for the ticket-pricing structure, considering the country is in the midst of an economic recession.
It has denied high ticket prices have put Brazilian fans off, while the International Olympic Committee said prices offered to overseas fans were not too high.
How the other venues fare
Velodrome: There were a few empty seats at the start of the second day of track cycling, but the venue was soon full and numerous Union Flags on display hinted at a big British presence.
Tennis Centre: The crowd for Andy Murray's match was disappointing but Rafael Nadal's quarter-final win against Brazil's Thomaz Bellucci generated the best atmosphere of the week from an almost full stadium.
Equestrian Centre: The team dressage final failed to attract a large crowd to Deodoro, with the centre less than half full all day.
Carioca Arena 3: It was a similar story for day seven of the fencing competitions, where the men's team final was poorly attended.
Shooting Centre: One of the few places to buck the trend was the Olympic Shooting Centre, where the men's 50m rifle prone final and women's skeet contest saw the stands almost full.
Lagoa Stadium: Rowing also drew a large crowd as a number of medals were decided, with a large British presence noticeable.
Reserva de Marapendi: Official attendance figures were not immediately available for day two, but more than 6,000 spectators watched golf's return to the Olympics on Thursday and Craig Connelly, caddie for Martin Kaymer, wrote on Twitter: "Crowds here in Rio for the golf have been fantastic. Hope it looks good on TV!"
Future Arena: Handball has proven a popular attraction for athletes from other sports as well as spectators, with the stadium about 85% full for Spain's clash against Romania in the women's competition.