Dog walking is lowering crime and making Merton a cleaner borough, Merton police have said.
A spokesman said their Merton Dog Watch scheme - which encourages walkers to report crime and anti-social behaviour - had lead to 21 crimes reported and one arrest made in its first four months.
Last week its 500th member, Claire Petherwick from Mitcham, was presented with a goody bag including prizes for her black Labrador Honey.
She said: "I really like the idea that just by going for our walk, Honey and I are doing our bit for the borough and fellow residents."
Chief Superintendent Dick Wolfenden, borough commander, said: "It is fantastic news that the scheme has grown so quickly."
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