In England you have an assessment with an AMHP (who is there to protect the rights of the patient) and two doctors. One is a specialist MH doctor with training on the law, the other knows the patient. They have to agree that the patient poses a serious risk of harm to themselves or other people, and that they must be detained in hospital for assessment or treatment. The AMHP asks the patient's nearest relative if they object to the section -- if they do object it becomes harder to detain someone.

You then provide free legal advice, and access to mental health act tribunals where people can challenge their detention.

You have independent regulators who provide some oversight, the rpess can report on it all, and of course the courts provide a final backstop.

There are problems with this, but it's pretty good.

We have short detentions for assessment. We have longer detentions for treatment. We have police powers to take someone from a public place to a place of safety for assessment. There's another police power where the police can take someone from their home to a place of safety for assessment, but only if they have a valid warrant.