Hackers occupy a scary place in the popular imagination — shadowy figures capable of taking down hospitals and causing international havoc, as happened in the massive WannaCry ransomware attack earlier in May.

But the reality can be far less malicious — and more mischievous.

Earlier this week, hackers managed to take control of a billboard outside a shopping centre in Liverpool.

And they took the opportunity to leave a remarkably polite message: "We suggest you improve your security." Sincerely, your friendly neighbourhood hackers."

Shoppers spotted the message, and posted photos of it to Reddit and Twitter.

A spokesperson for Liverpool One, the shopping mall, told the BBC that the screen was deactivated once they realised what happened. "Our screens are operated by an external company [Elonex] which is currently investigating what happened as a matter of urgency," they said.

"As soon as the apparent hacked message appeared on the screen, Liverpool One immediately closed it down."

Meanwhile, Elonex, the screen company, said: "We can confirm an incident occurred over the weekend on one of the 18 screens we operate at Liverpool One ... The incident appears to have been good-natured and not intended to cause offence or disruption, for which we are grateful."