Hundreds of randomly selected parking bays in Central London were free for the day to celebrate the 500,000th pay-by-phone transaction.
Drivers will be notified when they try to pay using their mobile
Westminster Council has chosen 200 bays that were exempt from charges on Thursday between 0830 BST and 1830 BST.
Motorists discovered if they picked a "golden bay" when they attempted to pay using their mobile phone.
The cashless scheme was introduced in June and now operates in about 2,000 bays across Westminster.
Westminster Council decided to scrap parking meters in favour of the pay-by-phone system after a successful pilot in 2006.
Registered motorists pay by inputting their details, via a phone call or text, which are transmitted to parking attendants' hand-held computers.
Westminster councillor Danny Chalkley, said: "The fact that we now have over 100,000 customers who have completed over 500,000 transactions is testament to the ease of use and convenience of the service to our customers.
"Because this service has been so successful, it makes sense that we increase the number of bays in Westminster that allow motorists to pay using their mobile phones."
Motorists need to set themselves up with a pay by phone parking account, either via a website at home or a mobile phone or SMS text on the street.
If they are delayed returning to car they can top up their parking fee without the need to return to the vehicle.
Pay-and-display facilities are available for those unable to use pay-by-phone.