Tickets for the 2012 London Olympics went on sale on Tuesday but some applicants found they were unable to process their orders through the online payment system.
Organisers denied there was a “glitch” with the website after sports fans with Visa cards which expire before the end of August were blocked from placing their orders.
London 2012 said instructions on the website and the ticketing guide clearly states that in order to process an application, Visa cards must expire no earlier than August 2011.
A London 2012 spokesman said: “It is being sorted. It is an issue with Visa rather than the website or our systems.”
Applicants were urged to approach the start of the ballot for 6.6 million tickets - coinciding with 500 days to go until the Games begin - as a marathon not a sprint, with applications possible for the next six weeks.
Tickets are not allocated on a first-come-first-served basis, meaning every request made until April 26 stands an equal chance of success.
London 2012 chairman Sebastian Coe told people there was no need to place their orders immediately.
“There will be no greater chance of getting a ticket if you apply on the first day rather than later,” he said.
Organisers are determined to avoid the problems that plagued the 2008 Beijing Olympics, whose website crashed within minutes of tickets going on sale.
As the end of the first day of sales approached, the site appeared to be dealing with the rush.
A giant fireworks display to mark the 500-day milestone was to take place in London on Tuesday after Olympic legends Carl Lewis and Nadia Comaneci came to the capital to help with celebrations.
But in one embarrassing incident, the showpiece countdown clock on Trafalgar Square stopped just a day after it was unveiled in a glitzy launch.
The steel electronic clock made by Omega ground to a halt at 500 days, seven hours, six minutes and 56 seconds, then began counting up instead of down before a technician hastily arrived to repair it.
Applicants for tickets from Britain and most European countries should go to www.tickets.london2012.com, but those from other countries will need to contact their local National Olympic Committee or authorised sellers.
All applicants will find out if they have been successful by June 24.
A quarter of the Games’ budget - approximately two billion pounds ($3.2 billion) - needs to be raised from ticket sales.
Coe revealed that 2.5 million people had registered their interest in buying tickets even before sales began, with more than one million expressing a desire to see football -- an event which traditionally does not fill stadiums at the Olympics.
He said organisers had taken every precaution to ensure that tickets are bought by people who want to watch the Games, not by unscrupulous buyers who will try to exploit demand for the top events by re-selling at a profit.
“No one will believe me if I say we will have a ticket tout-free Games -- but we have done everything we can to disrupt that business,” he said.
A specialist London police unit to combat scalping has already made arrests.
Tickets for sports events range from 20 pounds (32 dollars, 23 euros) up to 725 pounds for the top-rated athletics sessions such as the men’s 100 metres final when Usain Bolt could retain his title.
Organisers have dismissed suggestions that tickets are overpriced, noting that a third are priced at 20 pounds.
There are 650 sessions across 26 sports to choose from, with individuals limited to a maximum of 20 tickets each, although that drops to four for the most popular events.
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