Olympic organizers are investigating why many seats were empty during events at venues including the Aquatics Centre in east London.
Areas high in the stands at the sold-out event were full but several hundred more expensive seats lower down were not filled.
LOCOG said some accredited seats – for press and media – were empty.
Empty seats have also been seen at Wimbledon and the volleyball at Earl’s Court on Saturday afternoon.
A LOCOG spokesman said earlier: “We are aware that some venues have empty seats this morning.
“We believe the empty seats are in accredited seating areas and we are in the process of finding out who should have been in the seats and why they weren’t there.”
London 2012 chairman Lord Sebastian Coe has previously threatened to name and shame companies which do not use their tickets.
The Queen and the Duke of Edinburgh visited the Aquatic Centre during the morning session while 14-time Olympic gold medallist swimmer Michael Phelps scraped through to Saturday night’s final of the 400 m individual medley.
The number of empty seats was the only negative aspect of the session and it did not harm the atmosphere.
But it meant Olympic organizers still had some work to do.
Some blocks of empty seats have been observed at Wimbledon’s centre court.
One spectator, Rachel Clarke, said: “Since they were in prime position – near where the players came out and the royal box – I can only assume that they were corporate seats. They were in a good spot for a fantastic game but they remained empty.”
Many people have expressed disappointment over the sight of empty seats after failing to get tickets for events.
Diana Hill said: “To sit down and watch the first day and see the dressage event half full, huge chunks of seating empty in men’s gymnastics and badminton (and I’m sure many more events), is incredibly frustrating.
“Where are all these apparently <<sold out>> tickets going to? Sponsors? It’s a sad joke.”
Tennis fans have also complained over queues to get into Wimbledon on Saturday morning, saying a ticket office was closed because the key to open it had been lost.
Richard Till, from Birmingham, said he spent three hours waiting to collect tickets.
He said: “There were people visiting from abroad standing in the queue behind me; it was an embarrassment for the first full day of Olympics events. The people working in the ticket office didn’t have a clue; it took four people standing around a computer to print off a receipt.”
LOCOG has yet to respond to requests for a comment.