Three men have been arrested as a man remains in critical condition in intensive care.
The victim was attempting to break into cars when the attack unfolded, police believe.
Officers were called at 1.42am today (November 20) to Willowherb Road in Emersons Green .
Police received a report of a man having been detained by a member of the public after allegedly trying to break into cars.
Officers attended by 1.51am and found the man to be unresponsive. An ambulance arrived at 2.05am and the victim was rushed to hospital where he remains in intensive care.
Three men aged 25, 35 and 49 have now been arrested on suspicion of GBH and remain in police custody.
A force spokesperson said: "An investigation is under way and our enquiries are ongoing.
"If you have information about the incident and have not yet spoken with officers, please call 101 and give the call handler the reference 5219628280."
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