Rockstar have said that GTA online will be available from around 12 noon (UK Time) today.
Rockstar have been buying extra servers to try and cope with the demand at launch.
The online portion will be added via a small update and can be accessed via the bottom character select slot.
PSN in Europe will be down between Wednesday September 25 at 3pm until Thursday September 26 at 3am for maintenance.
"For the duration of the maintenance you will be unable to log into the PlayStation Network, Account Management and the PS Store. PS Home will also be unavailable.
Some users may be able to sign in however, the services listed above will still be unavailable during the maintenance window.
There may be some additional downtime after the maintenance period for testing purposes. "
So it appears that you can't sign into PSN before the downtime and still play online like you usually can.