Officers from West Midlands Police were called to Priory Queensway in Birmingham around 3.30pm after reports of violence involving a number of people.
It is believed that one was carrying a machete and a 16-year-old boy was taken to hospital with knife wounds not believed to be life-threatening.
Officers are viewing CCTV footage to find the people responsible, but have said they believe it to be an isolated incident.
A 15-year-old boy has been arrested on suspicion of attempted murder, as detectives at West Midlands Police work to identify others involved in the violence, and remains in custody to be questioned.
The area around Priory Queensway was closed in both directions and traffic queued from Colmore Plaza to Old Square.
National Express bus services were diverted because of the incident and representatives from the company were stationed at Priory Queensway to redirect people waiting for their buses.
A spokesman for West Midlands Police said: “We’re investigating after a teenager was stabbed in Birmingham city centre this afternoon.
“We were called to Priory Queensway just after 3.30pm to reports of violence involving a number of people.
“It’s believed one was carrying a machete, and a 16-year-old boy was taken to hospital with knife wounds which are not believed to be life-threatening.
“Officers have already reviewed CCTV and are urgently trying to identify and arrest the suspects.
“At this stage, it’s believed to be an isolated incident involving people who were known to each other with no wider threat to the public.
“We remain at the scene tonight while the investigation continues.
“Contact us via Live Chat at west-midlands.police.uk, quoting log 2623.”
Det Sgt Laurence Green said: “The area was really busy with shoppers at the time and we know hundreds of people in the area at the time saw what happened.
“I’d ask anyone who’s not yet spoken to us to get in touch via Live Chat on our website or Crimestoppers anonymously.”
West Midlands Ambulance Service have been asked for further information.