Police said the victim, in his 30s, collapsed in Lower Parliament Street, Nottingham, just before 2am after being assaulted in nearby Broad Street.
The area has been cordoned off and that is expected to remain in place for most of the day.
Nottinghamshire Police said patrols had been increased in the area to reassure local residents.
Det Insp Mark Parish said officers were “keen to speak with anyone who may have seen an altercation between a group of men”, some of whom were believed to have fled along Old Lenton Street.
More news from Nottingham
