World Record Gaming Gathering.
They narrowly broke a record of 337 set last week in Germany.
Festival director Duncan Best said: "It was a bit touch-and-go for a while but the gaming community really stepped up to the challenge."
The record was verified by Guinness World Records at the Excel Centre in Docklands on Saturday.
A further 50 fans in fancy dress could not take part, however, as rules stated that characters who were primarily famous from movies were disqualified.
"So we had people turning up dressed as Lego Batman, for example, who could not be counted," said Mr Best.
Get your details right, BBC! We all know Batman is most famous from the TV show...
Still, it would have been nice to see the Lego Batman costume.
London Games Festival
Guinness World Records