Section 136

This section applies if it appears to a police officer that you have a mental disorder and are "in need of immediare care or control", they can take you to (or keep you are) a place of safety. You will be kept in the place of safety you were taken to so that you can be examined by a doctor and interviewed by an approved mental health professional, and any necessary arrangements can be made for your treatment or care.

You can be kept under this section for up to 24 hours (this can be extended up to 36 hours in some circumstances).