The network confirmed on Twitter that its users are 'unable to access data services' and 4G coverage.
Their technical teams are working on the issue as a 'high priority' and updates will be given throughout the day.
Providers such as Tesco Mobile and Giffgaff, which use O2's network, are also affected.
Customers can still make calls but cannot use applications that rely on a data connection or browse the internet. Some reported being unable to send SMS messages although this is unconfirmed.
Public transport users in Leeds voiced concerns on social media that they would not be able to display their travel passes if apps were down.
Other customers complained of having to drive without being able to access the satnav function on their phones, while some even demanded refunds from O2.