There have been teething problems as the online registration began for London's new bicycle hire scheme, Transport for London (TfL) said.
A "small number" of people were unable to enter their credit card details on the website on Friday, TfL admitted. It said it was fixing the problem.
About 6,000 bikes are being parked in 400 locations across central London.
But only those who register can use them in the first month - the bikes were originally to be available to all.
TfL said this was so it could "learn how members engage with the scheme, including patterns of bicycle usage".
Customers must activate an electronic key, which costs £3. Membership rates range from £1 for 24 hours to £45 for a year.
Users can take a bike for 30 minutes for free, but must pay rental charges if they use it for longer than half an hour.
The company said about 2,000 people had successfully joined the scheme by Friday afternoon.
"A small number of potential members have been experiencing difficulties with the card details section of the registration form," TfL said in a statement.
"Software engineers will be updating the system in order to resolve the issues and enable any potential members who have been unable to sign up today to do so successfully tomorrow [Saturday].
"We apologise for any inconvenience that this causes."