Police have closed off Elm Grove in Brighton following an assault last night.
A man in his 20s was seriously injured in the assualt, which happened at 9.30pm, police said.
The residential area has been closed off for several hours while officers deal with the incident.
Officers are asking witnesses to report details online or telephone 101, quoting Operation Dymchurch.
Alternatively they can contact Crimestoppers or phone the independent charity anonymously on 0800 555111.
A spokesman for Sussex Police added: “Enquiries are at an early stage and currently there are no further details.”
Brighton & Hove Buses has confirmed services have been diverted following the incident.
A spokesman has since said it is likely services will be diverted for the rest of the day.
A murder investigation has since been launched. Read our updated story: Murder investigation launched after man stabbed in Brighton