Olympic organisers have launched an investigation after fans expressed disappointment at the sight of rows of empty seats on the first day of the London Games.
Locog were prompted to act after gaps were visible at a number of venues, including the Aquatics Centre where British medal hope Hannah Miley missed out on a podium place in the 400 metres individual medley final.
In contrast, huge crowds lined the streets, where tickets were not required, to watch Mark Cavendish and Team GB compete in the cycling road race.
A spokesman for Locog said the majority of the empty seats were believed to belong to accredited groups such as governing bodies and the media.
He added that some tickets remained unsold and urged members of the public to check online for availability.
"Many of our venues were packed to the rafters today. Where there are empty seats, we will look at who should have been sitting in the seats, and why they did not attend," he said.
"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."
Culture secretary Jeremy Hunt said the empty seats were "very disappointing" and suggested they could be offered to members of the public.
"I was at the Beijing Games, in 2008, and one of the lessons that we took away from that, is that full stadia create the best atmosphere, it's best for the athletes, it's more fun for the spectators, it's been an absolute priority," he said.
"I think it was accredited seats that belonged to sponsors, but if they're not going to turn up, we want those tickets to be available for members of the public, because that creates the best atmosphere."
He said a system had been introduced for these Olympics similar to the one used at Wimbledon, where people coming out of the stadium handed on their tickets so the seats could be made available to other people.
"So we are trying a lot of innovations, it's a shame this happened, but we are going to do everything we can to make sure we fill up these stadia," he added.
Gaps in the seating were also visible at the gymnastics, handball, volleyball, badminton and basketball arenas.
The cheaper seats higher in the stands were mostly full but those lower down, which are generally more expensive, were not filled.
One basketball spectator, Jane Smith, from London, said: "It's very disappointing to see this, particularly as we all tried so hard to get our tickets. It doesn't help the atmosphere at all."
Other fans took to Twitter to express their concerns.
One fan wrote: "All those empty seats should have been given to the locals or sold on first come first serve on the day. Games are looking real empty!"
London 2012 chairman Lord Coe has previously threatened to name and shame companies which do not use their tickets.