It is thought the seats could have been allocated to VIPs or members of the media.
BBC News website readers have been telling us that other Olympic venues have had similar issues.
Paul Mullins, Geneva
I have a story not about empty seats, but non-existent seats!
I flew over from Switzerland and rocked up at Earl’s Court for the Olympic volleyball on Saturday with five of my family members.
We found that three out of our six seats didn’t actually exist. There was just empty space where the seats should have been. We were faced with sitting on the floor!
Nobody was able to help us. An IT guy tried, but there wasn’t much he could do.
We spotted a whole block of empty seats, but we were told it was for a Russian delegation. Needless to say, no-one turned up and they remained empty.
In the end, we sat separately in whatever seats we could find. This was a shame – the whole point of going to watch sport together is to share the experience and bounce things off each other.
Ann Smith, Australia
My husband’s just devastated – he was crying yesterday.
We’ve travelled over from the Gold Coast in Australia to see the Olympics, but our tickets for the boxing haven’t shown up.
We’ve both been had for £180 on boxing tickets, but I’m sure there are others who have been had for much more.
When my sister called 118 for the contact number of the website we bought tickets from, they told us there was a note on the system saying it was a bogus ticketing website, and that over 100,000 people had got in touch asking for the same information.
We’re staying at Brixham, in Devon. Everyone here is feeling sorry for us. We’re here for five weeks and the Olympics was the main reason for coming over.
Paul Fondie, Kew
I attended the 11:00 to 13:00 gymnastics session. We managed to get tickets in the last couple of weeks.
The entire upper section of seats was full, however in the two main areas of the lower section, there were quite a few empty seats – probably around 35% empty.
My wife and I acquired single seat tickets but managed to get a few people to switch seats so we could sit together.
We wanted to bring my six-year-old son along but couldn’t get an extra ticket.
There were a lot of discussions among the crowd about the empty seats and problems facing families who couldn’t get seats together.
It tainted my experience of the Olympics – it was our moment to come under the microscope and show that London can do it well.
It was great to see the GB men’s gymnastics team do well against China, however, it was a shame not more people could see it happen live.
Jane Henderson, Leicestershire
I have been trying constantly for two days to purchase some tickets for either the Olympics or Paralympics.
I need a wheelchair space so have to ring a 0844 number which constantly tells me they are busy and tickets can be purchased online – but I can’t do that as I need a wheelchair space.
Other times you get as far as choosing your wheelchair space, only to be cut off.
It’s very frustrating, especially seeing all the empty seats on the TV.
I think the lines must be flooded with people whose tickets haven’t arrived in the post yet, because you hear a pre-recorded ‘don’t panic’ message to them.
We couldn’t afford tickets before but having got married in June we have some wedding present money and want to come with my children as a treat. We’d really love to see the Paralympic athletics.
I just hope the tickets don’t run out before we get through.
I went to the Olympic swimming on Saturday evening. It was absolutely fantastic.
Yes there were some empty seats but these were clearly for the Olympic family and the media. What matters is that the public sections were packed.
The atmosphere was fantastic. Enormous cheers, wonderful venue and some records smashed.
What made the experience for me was the enthusiasm of the volunteers and the soldiers on duty.
Smiles, encouragement and beaming faces everywhere. At countless national and international events I have never seen anything like it.
Well done London. It made me exceptionally proud of our Games and our city.
Christopher Redburn, Dorchester
I was on Centre Court at Wimbledon for the tennis.
The first three games of the day were really well-filled – those were the ones advertised the night before. But then later in the day it seemed they had added a fourth game.
There were lots of empty seats, many due to sponsor seats remaining empty, but also many individual seats. I could tell which were which because the sponsor seats have a coloured sticker on them.
The nationalities of the players weren’t advertised prominently enough, so people didn’t stay on. It must have been three-quarters empty when British tennis player Anne Keothavong came out.
It was embarrassing how little support we provided.
And unlike at the Wimbledon championships, there seemed to be no system whereby people leaving the venue could put their ticket in a box so that newcomers could come in to take up the empty seats.