In a tweet the bank said it was "urgently investigating" the cause of the problem.
Customers have been contacting the bank on social media to report problems accessing their money and making payments.
A Barclays spokeswoman said: "This was a fault with our internal systems and not an external cyber-attack."
"We can confirm that customer services are now recovering from a technical issue we experienced this afternoon impacting some of our customers," the spokeswoman added.
The bank has tweeted that customers "should be able to make payments & access ATMs" and that online and mobile banking are "back online".
Twitter user @BenTavill said: "@BarclaysUK you need to fix your systems, need to withdraw cash but can't. Not good at all."
Barclays is looking into "technical issues" which appear to be affecting debit card users.