Hackers have obtained personal email addresses from Sussex Police.
Officer have launched an investigation into security breaches of its external website.
Amaraghosha Carter, joint head of IT for Surrey and Sussex Police, said: “Those responsible have obtained email addresses of a number of officers and personal email addresses of some members of the public who have used the services of our website.
“Communications staff have contacted around 270 people who may have been affected, in particular to give them security advice in relation to their passwords for our community messaging service.”
She added: “Our website is entirely separate to those systems used to investigate crime. The activity has not impacted on any other force IT, web or telephony systems and operational response is unaffected.
“There has been no impact on our service to the public. Measures are being put in place to ensure that the security of the website is not compromised further.
“Work is being carried out to ensure that all our IT systems remain resilient and secure and an investigation is under way to find those responsible.”
Anyone concerned can contact Sussex Police on 101 or email email@example.com quoting serial 1483 of 23/12.