A man of 21 has been arrested on suspicion of murder after a 43-year-old man died in a street fight last night.
He fell to the pavement, hitting his head, in Newick Road, Moulsecoomb, Brighton, at 6pm.
Witnesses said he got up and walked away but collapsed minutes later after leaving the street.
He was taken by ambulance to the Royal Sussex County Hospital, Brighton, where he died.
A post-mortem examination was being carried out this afternoon to determine the cause of death.
Police were trying to piece together what happened during the incident.
Detectives activated a major incident suite at Sussex House in Crowhurst Road, Hollingbury, Brighton.
A spokesman said: "We are treating this as a major crime."
The identity of the dead man was being withheld until relatives have been told.
Detective Chief Inspector Reg Hooke, the force senior investigating officer in charge of the inquiry, said: "Police are still trying to establish what happened in Newick Road last night.
"Tragically a man has died after what appears to be an altercation."
Police cordoned off about 30 metres of road with tape while officers carried out forensic tests, questioned witnesses and made house-to-house inquiries.
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