Earlier, a message told users the site was experiencing high demand and to "try again later".
Some frustrated would-be buyers vented their feelings on Twitter.
In a statement the London Organising Committee of the Olympic Games (Locog) said the site had not crashed, but had been slowed by heavy demand. It said the site was still processing applications, albeit more slowly.
There are 6.6 million tickets in all, but about half will go to corporate and VIP guests.
Applications will be processed in May and June and those who have been successful will be notified by 24 June.
Organisers said people should ensure they had money in their account to pay for every event they have applied for.
Before tickets went on sale on 15 March, more than 2.5 million people had signed up to the official website.
'Massive surge' Standard prices range from £20 to £2,012 - the top ticket for the opening ceremony - and oversubscribed events will be decided by a random ballot.
There has been criticism of the prices, the ballot system and the fact money is taken from accounts before applicants know which events they have secured.
But organisers said for more than 200 sessions, children under 16 paid the same number of pounds as their age, and those over 60 were charged £16.
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He said: "Overall, 75% of the tickets there are for public application. That's the rough split. But... for some of the bigger events, that will come down." Continued