London -Athletics was one of 17 sports that sold out of tickets for the 2012 London Olympics after fans flocked to the official website for the start of the second round of sales on Friday.
Tickets remained for just five sports -- football, volleyball, wrestling, weightlifting and boxing -- after fans desperate piled on to the website when it opened for business at 6:00am local time on Friday (0500GMT).
Athletics, basketball, handball, judo and rowing were among the sports that sold out.
Organisers said they were unable, at this stage, to say how many tickets had been sold on Friday, although some intitial estimates suggested more than 500,000 were taken up during the day.
Only the 1.2 million applicants who failed to get any tickets in the first round of sales were eligible to apply.
Some 2.3 million tickets went on sale on Friday, mostly for the men's and women's football tournaments, after three million tickets were sold in the first round from March 15 to April 26.
Organisers said the website was working, despite numerous reports of delays.
A London 2012 spokeswoman said: "The system is working, it has not crashed. Due to high demand some people are being held outside the system.
"For those in the system it is working and transactions are going through, but we are having to hold some people back from going through the site due to high demand."
One applicant said: "I was quite excited when I got in right on 6:00 am and was able to select hockey and archery sessions but confirming the application was very frustrating. It took me about 12 goes."
And there was fresh frustration for many fans, who found themselves directed to a webpage saying: "We are experiencing high demand. You will be automatically directed to the page requested as soon as it becomes available. Thank you for your patience."
Even some of those who got beyond that point, to the stage of requesting tickets and providing bank details, were told: "Sorry we cannot process your request. Please try again later."
The London 2012 spokeswoman added: "We had an initial slowdown when a lot of people were held out of the system but after those people went through it went pretty steadily."
The disappointed 1.2 million applicants have the exclusive right to buy tickets in a 10-day window in the second round, but the process this time is on a first come, first served basis.
Those who were successful in the first round will get another chance to buy tickets from July 8 to July 17. Another round of ticket sales will take place early next year.