If a constable finds in a place to which the public have access a person who appears to him to be suffering from mental disorder and to be in immediate need of care or control, the constable may, if he thinks it necessary to do so in the interests of that person or for the protection of other persons, remove that person to a place of safety within the meaning of section 135 above.


A person removed to a place of safety under this section may be detained there for a period not exceeding 72 hours for the purpose of enabling him to be examined by a registered medical practitioner and to be interviewed by an approved mental health professional and of making any necessary arrangements for his treatment or care. 


See more detail here.

See IOPC guidance.