Four people are being treated for injuries after several stabs at Arndale in downtown Manchester.
Trams also stopped running at Exchange Square at the entrance to the shopping center.
The Northwest Ambulance Service said it was treating four people after being called at 11:17.
Messages on social networks showed a large number of officers at the scene and the suspect, who was hit by a stun gun.
Greater Manchester Police (GMP) said that so far there have been no reports of any deaths.
The image posted by Manchester Evening News seems to show a patient being treated at the Starbucks Cafe in the center.
Several people posted on Twitter that they were evacuated.