A man has been arrested after a man was stabbed while trying to perform a citizen's arrest in Melbourne's CBD this morning.

A knife-wielding man was seen acting erratically as he ran through a car park on Lonsdale Street, near Melbourne Central, about 6am, police allege.

Two men in hi-vis chased him down Elizabeth Street before tackling him to the ground near the intersection of Lonsdale Street.

They held the man, a 32-year-old from Dallas, down until police arrived three minutes later and took him into custody.

One of the bystanders, a 41-year-old Maddingley man, was stabbed in the lower body, according to police.

He was taken to St Vincent's Hospital with non-life-threatening injuries.

The other man, a 35-year-old Sanctuary Lakes man, was uninjured.

Police have increased p

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