Olympic organisers are investigating why many seats were empty during events at venues including the Aquatics Centre in east London.
The BBC's Nick Hope said 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 were also seen at Wimbledon, the volleyball at Earl's Court and the gymnastics at the O2.
A Locog spokesman said: "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.
"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."
BBC Olympics correspondent James Pearce tweeted on Saturday night that more tickets had become available on the London 2012 website, including to athletics and swimming.
London 2012 chairman Lord 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 400m individual medley.
Sports correspondent Nick Hope said the number of empty seats was the only negative aspect of the session.
It did not harm the atmosphere, which at times - particularly around the British performances - was deafening, he said.
But it meant Olympic organisers still had some work to do, he added.
Some blocks of empty seats have been observed at Wimbledon's centre court.
One spectator, Rachel Clarke, told the BBC: "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."
There were also many empty seats at gymnastics on Saturday morning, Stephen Morris said. "There was a good atmosphere but it could/would have been so much better if the main part of the arena had been as full as the others," he said.
Many people have contacted the BBC to express 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 to the BBC 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.
Within the Olympic Park there is a booth where tickets are resold once the seats are vacated, Locog said.