It appeared that the more expensive seats at several venues, including Wimbledon, Earl's Court for the volleyball, and gymnastics at the North Greenwich Arena, were empty throughout the day.
In the morning session at the Olympic pool, there were also numerous unclaimed seats.
The sight of half-full venues infuriated those fans who applied for tickets in a controversial ballot system, only to miss out on attending any of the Olympic Games.
One of London's key objectives was to avoid a repeat of Beijing, where several sessions were officially sold out, yet not played out to full houses.
By contrast, an estimated million fans lined the streets to watch the Olympic men's road race.
Locog said that some of the accredited seats which should have been claimed by the media were empty, but a spokesman confirmed an investigation would take place.
"Many of our venues were packed to the rafters today," said the spokesman. "Where there are empty seats, we will look at who should have been sitting in the seats, and why they did not attend.
"Early indications are that the empty seats are in accredited seating areas, but this is day one, and our end of day review will provide a fuller picture of attendance levels across all our venues."
- Sports & Recreation