About 2.5 million people a day are fighting to get their hands on Olympic tickets with pressure on the ticketing Web site running right through the night, organisers said.
Demand appears to have been boosted by Team GB's strong performance in the medals table and the start of the track and field events in the main stadium on Friday.
Frustration with an online system that has been unable to cope with demand has been compounded by television footage of empty seats in so-called accredited areas set aside for Olympic officials, sports federations, national Olympic committees, athletes and the media.
London Olympic organisers (LOCOG) have asked those who do not intend to use their seats in these area to give them up, and have trimmed the seating in an effort to make more tickets available to the public.
But one in five of the accredited seats remained empty on Thursday.
"A lot of them are applying for the same things, so obviously as they get to the check-out, they find that someone else has pipped them to the post and the product is not there that they've ordered," a LOCOG spokeswoman told reporters on Friday.
She said 100,000 tickets had been sold during a 48-hour period, but tens of thousands remained.
"So it is about persistence," she said.
"People are going into the system, they are successfully buying tickets, they are successfully attending the Games.
"But the sheer volume of people on the site, all through the day, and all through the night, by the way, is vast."
LOCOG has reclaimed thousands of accredited seats, including 2,400 for athletics, and sold them within hours.
The last-minute sales have boosted the occupancy rate in accredited areas from 60% on Tuesday to 81% on Thursday. No figures were available for earlier in the week, when vacancies were at their highest.
About 75% of the total 8.8 million tickets were sold to the British public. Spectator occupancy in all venues was 92% on Thursday.
LOCOG put another 275,000 tickets on sale this week, mainly for soccer, which became available after logistics such as TV camera positions were sorted out.
Friday and Saturday are expected to be the busiest days so far at the Games, with 200,000 people in the Olympic Park, up from 135,000 on Thursday.
Westfield Stratford City shopping mall, the gateway to London's Olympic Park, will be shut on these days to people without tickets to try and manage the crowds.
