Canal Street was shut between Carrington Street and the entrance to Broadmarsh car park, including the pavement outside Nottingham Crown Court all the way down to Trent Street.
A second cordon was in place outside Loxley House, the headquarters of Nottingham City Council.
Ch Insp Craig Berry said: “This assault took place in a busy location and we believe there may be witnesses or people with recorded footage of the incident.
“We would urge them to get in touch without delay.”