Police forensic teams are working within a cordon on Thurland Street in the city after an incident in the early hours of 1 January 2023.
Police say the incident, in which two people were stabbed, happened at around 4 am this morning Sunday 1 January.
A 24-year-old man was taken to hospital with serious chest injuries.
The incident happened outside Mojo nightclub in the city.
A police cordon is in place at Thurland Street and on Clumber Street at Sports Direct.
In an appeal for witnesses, Detective Superintendent Hayley Williams, of Nottinghamshire Police, said: “This was a very serious assault that left two young men in hospital.
“We are working hard to understand what happened in the moments before, during and after this incident and are asking for any remaining witnesses to come forward.
“We would also like to speak to any taxi or bus drivers who may have may have noticed an injury to one of their passengers, or to anyone else who has noticed unexplained injuries to someone they know or blood stains to their clothing.
“Several police cordons are in place across the city centre in relation to this incident. Whilst we appreciate these may cause disruption to local businesses, it is essentially imperative that we gather all available evidence about what happened in the early hours of this morning.”
Anyone with information is asked to call 101 quoting incident 240 of 1 January 2023. Alternatively, they can speak directly to a police officer.