At least five people have been treated after a stabbing incident in a shopping centre in central Manchester.
Greater Manchester Police said officers were responding to the incident in the Arndale Centre.
North West Ambulance Service said it had treated four people after being called at 11.17am to reports of a stabbing incident.
Social media posts showed a large number of officers at the scene and a suspect being Tasered.
An image sent to the Manchester Evening News appeared to show a patient being treated by a Starbucks cafe in the centre.
Meanwhile, footage posted to social media showed police officers running through the shopping centre and several emergency services vehicles parked outside.
Several people posted on Twitter to say that they had been evacuated.
A shop worker, who gave his name only as Jordan, 23, told the PA news agency: "A man was running around with a knife lunging at multiple people, one of which came into my store visibly shaken with a small graze.
"Soon after, security staff told all retail staff to close their doors and move the public to the back of the stores."
Jordan, who has only been working at the centre for a few months, said the attack took place on the ground floor.
A Greater Manchester Police statement said: "We are currently responding to an incident at the Arndale Shopping Centre in Manchester City Centre.
"Officers are in attendance and early enquiries suggest that several people have been stabbed.
"A man in his 40s has been arrested on suspicion of serious assault. He has been taken to custody for questioning.
"Enquiries remain ongoing."
Freddie Houlder, 22, from Market Drayton, was in the Arndale Centre when he heard "a load of screams just outside" the shop he was in.
He said a woman then came into the shop and told others "a guy just ran past the shop and tried to stab me".
Mr Houlder added: "Luckily she had quite a thick jacket - she thought originally it was a fake knife because of how easily it grazed off but police came in and said it was a real knife and she burst into tears."
He said he was evacuated several minutes later at which point he saw "a bunch of curtains covering someone seriously injured or dead, I don't know".
"I definitely don't think it was gang violence because this guy was going around trying to stab random people, I believe."
Counter-terrorism police are leading the investigation into the stabbings at the Arndale shopping centre because of the location and nature of the incident, Greater Manchester Police said.
A GMP statement said: "Specialist officers are continuing to respond to an incident at the Arndale shopping centre in Manchester city centre.
"We can confirm that five people have been stabbed and taken to hospital.
"In these early stages we are keeping an open mind about the motivation of this terrible incident and the circumstances as we know them.
"Given the location of the incident and its nature officers from Counter Terrorism Police North West are leading the investigation as we determine the circumstances.
"A man in his 40s - who was arrested at the scene on suspicion of serious assault - remains in custody for questioning."
The Exchange Square entrance to the Arndale Centre remains cordoned off.
Members of the public are not being allowed to proceed along Cross Street from Selfridges onwards.
Four armed officers are stood on guard nearby.
Three ambulances left the scene before a fourth arrived.
Members of the public were later asked to move back to the junction with Market Street.
A fourth ambulance has left the scene as a police helicopter continues to hover overhead.