Some police officers' email addresses have been obtained following cyber attacks on a force's external website.
Sussex Police said personal email addresses of some members of the public who have used the website have also been obtained.
Three breaches which have been identified within a "contained area" of the force's external website could be linked, a police spokesman said.
Around 270 people who may have been affected by the cyber attack have been contacted by the force about boosting their online security.
The spokesman said the website is "entirely separate" to the systems the force uses to investigate crime, and there has been no impact on its service to the public.
In the meantime, an investigation has been launched to find those responsible, and efforts were being made to prevent further attacks.
Amaraghosha Carter, joint head of IT for Surrey and Sussex, said: "A full investigation is under way to identify the source of the breaches and their impact.
"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.
"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."