The Metropolitan Police has been criticised for doing "unwarranted" strip searches on suspects and not properly recording use of force.
An inspection of 15 custody suites in London found more than 10,000 people were strip-searched in a 12-month period, amounting to 16% of detainees.
HM Inspectorate of Constabulary, Fire and Rescue Services (HMICFRS) said this was "higher than we normally see".
The Met said strip searches were not carried out "as a punitive measure".
A spokeswoman added the searches were "a safeguarding measure to protect not only officers working in our custody suites, but individuals coming into custody and those already detained".
"They are also a vital power used to identify and seize evidence," she said.
The HMICFRS and HM Inspectorate of Prisons joint inspection, which took place in July last year, did find some "positive features" such as respectful treatment of detainees.
But the inspection report said it was concerned not all strip searches were "warranted or properly justified", while a "high proportion" of children and a "disproportionately higher" number of black and minority ethnic suspects were subject to the searches.
Inspectors also expressed concern about how long some detainees remained in spit hoods.
The report said: "Not all the strip searches that we saw during the inspection were warranted or properly justified.
"The governance and oversight of the use of force in custody were not adequate to ensure that all use of force was proportionate and justified for the risk or threat posed."
"Not all use of force in custody suites was recorded, or the force used was not always accurately reflected in the custody record. Not all staff involved in use of force incidents completed individual use of force forms."
Inspectors referred two use of force cases for full review.
In a recommendation, the report said all use of force in the custody suites should be recorded, accurately reflect the force used, and be fully justified on the custody record.