Mental Health Act allows compulsory admission and treatment of people who:

1. Have a mental disorder of a nature and degree that warrants treatment in hospital, and

2. Need to be admitted in the interests of their own health or safety, or for the protection of other people.

Compulsory admission for assessment/for treatment is arranged using the appropriate section of the Mental Health Act:

Section 2 allows compulsory admission for up to 28 days for assessment.

Section 3 allows compulsory admission for up to 6 months for treatment (in people with an established diagnosis).

Sections 2 and 3 require an application from
Approved Mental Health Professional (AMHP, formerly an Approved Social Worker), or, rarely, the person's nearest relative,
Two doctors; one of whom is section 12-approved (usually a psychiatrist) and one who has previous acquaintance with the person (usually the person's GP if at all practicable).

Section 4 is used in exceptional cases to permit compulsory admission for up to 72 hours if there is urgent necessity, and undesirable delay would occur while trying to arrange admission under section 2. Section 4 requires an application from
AMHP (or, rarely, the person's nearest relative)
One doctor, preferably from a doctor with previous acquaintance (usually the person's GP).

Section 136 is used by police to take people from a public place to a place of safety to enable examination by a registered medical practitioner and interview by an AMHP.
The person's GP, where known, may be informed. The maximum period a person may be detained under section 136 is 72 hours.