The portal will allow fans who hold tickets for events they no longer wish or cannot afford to attend the chance to sell them on to other members of the public at face value on a first come, first served basis.
Should any tickets made available remain unsold in the first phase, the ticket holder will have them returned to their account
The site, which can be accessed via www.tickets.london2012.com, will be open until 3 February and there are plans to open a second resell window in March or April.
London 2012 organisers Locog have cautiously warned those left frustrated and empty-handed first time around not to expect a flood of tickets to become available.
The final consignment of 1.2m tickets across all sports is due to go on sale in April once the final seating plans within venues have been finalised.
The resale window opens just 48 hours after Locog was left red-faced, following an embarrassing, large-scale administrative error.
Around 3,000 customers were contacted and asked to return tickets for the synchronised swimming event, after a human data input error resulted in London 2012 chiefs putting an extra 10,000 non-existent tickets up for sale.