Kent Police's assistant chief constable said some people with serious mental health issues had waited more than eight hours to be seen.
The force's chief constable has asked health officials to address the issue.
The mental health trust for Kent said it was working with the police to try to limit the duration of assessments.
Kent and Medway NHS Social Care Partnership Trust said it aimed to provide "an effective and responsive service" to ensure that patients were seen swiftly.
'Place of safety'
However, when people were under the influence of drink or drugs, the assessment had to be postponed until the effects had reduced, it added.
In a statement, it said in the past year the trust had assessed 1,032 people at its five units across the county but "on occasion, people may have to be assessed in other locations, and these may include police custody suites".
Kent Police said it had detained 34 people under the Mental Health Act between June 2012 and the end of last month - 13 of whom were held in police custody suites or cells for more than eight hours as they waited for specialist NHS teams to assess them.
Assistant Chief Constable Paul Brandon said: "We have to take them to a police station as a place of safety, albeit a last resort we seem to be increasingly using them.
"And then obviously when they are there we have responsibility because of their vulnerability, almost in most cases, to give constant supervision, tying up police officers whilst we wait for the various assessments to be conducted."