More than 500 people with mental health issues in the Thames Valley were locked in police cells in the last two years due to a lack of NHS resources.
Police can detain people under the Mental Health Act if there is no "safe space" in hospitals available.
Health minister Norman Lamb has spoken out against the practice in the past calling it a "scandal".
Thames Valley Police said a pilot scheme involving mental health nurses out on patrol had reduced these cases.
July 2012 to June 2013 figures from the force show the amount of people detained in police custody under Section 136 of the Mental Health Act was 286.
'I was scared'
In July 2013 to June 2014 the figure was 242 - totalling 528 over the two-year period.
Claire Greaves, from Banbury, Oxfordshire, was detained in police custody after feeling suicidal.
She said: "I was scared. I think they treated me as well as they could.
"But I don't think it's the right place for people with mental health problems."
Mr Lamb has said he wants police forces to halve the amount of such cases by April 2015.
He has since implemented a pilot scheme across nine forces where officers take a mental health nurse out on patrol to professionally assess a person's need.
Thames Valley Police, which is part of the pilot, said its 2012 to 2014 figures showed a 15% reduction in cases, which was largely down to improved communication with the region's mental health services.
The force's mental health lead, Insp Jan Penny, said when she joined five years ago the majority of people detained by police under mental health legislation "would have gone into police custody".
She cited as an example June 2014 figures showing 91% were now referred directly to a so-called "safe space" where those with a mental health issue could be assessed and avoid being taken into police custody.
Currently NHS Foundation Trusts covering Berkshire, Oxfordshire and Buckinghamshire have nine such safe spaces but no plans have been revealed for an extension of that number.
Oxford Health NHS Foundation Trust said: "In the past year, with increased demand for mental health services we've doubled the number of places of safety we provide."
A spokesman added that apart from providing the mental health nurse as part of the pilot, it also provided "24/7 mobile phone advice and support to police who may be involved with incidents where individuals appear to have mental health issues".