Police are to be given access to specialist staff 24 hours, seven-days-a-week, to support people with mental health problems.
A new one-year pilot scheme will be trialled by Met officers in Lambeth, Lewisham, Croydon and Southwark.
In Lambeth and Southwark, mental health professionals will also be available to accompany police on visits.
A spokeswoman for NHS England London said the service plans to roll out the project across London.
The Mental Health Police Triage Service for London comes after the introduction of a separate scheme which saw mental health nurses being based in some police stations and courts.
Police in the four boroughs will have access to a 24-hour helpline manned by mental health professionals, who can give advice to officers attending call-outs and also advise if the person being dealt with is already known to the profession.
The pilot was started in April and launched officially earlier, and will run until April 2015.
Kate Davies, NHS England’s head of health and justice commissioning, said: “Many people who come into contact with the police have not committed a crime but are simply ill, and they are often going through some of the most difficult times in their lives.
“This scheme is about supporting police officers to make the right decisions about how to manage these situations, whether that means taking somebody to a place of safety or assisting them in the community with the help of a mental health professional.”