The Manchester Arndale centre has been evacuated after multiple people were 'randomly stabbed' inside.
Pictures and video on social media show large numbers of officers attending on the Exchange Square side of the shopping centre.
A photo shows a man lying face down while police point Tasers at him.
Two attackers were randomly stabbing shoppers, the Manchester Evening News reported.
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It is not known whether there are any fatalities.
Emergency services said four people have been injured.
Dramatic footage taken outside the Manchester city centre shopping mall shows a police officer deploying a Taser on a man.
North West Ambulance Service said it had treated four people after being called at 11.17am to reports of a stabbing incident.
A large police cordon has been put in place as police secure the scene.
Although the centre has been evacuated there are reports that people have been kept inside the Arndale.
John Sheerhout, a journalist for the Manchester Evening News, reported being told that two people were randomly stabbing people.
"I’ve just seen paramedics wheel a stretcher into The Arndale," he posted.
"I’m told that two people were ‘randomly stabbing people’ in the shopping complex.
"One suspect was arrested close to where I’m standing, I’m told."
Greater Manchester Police confirmed that they were responding to the incident.
"Just outside Arndale now. Shopping centre and several shops on Market street evacuated," one man Tweeted.
"Police officer tells me there’s been “an incident” and it’s been evacuated “for public safety” but wouldn’t say what’s happened."
A man who was leaving the Football Museum when the incident unfolded reported that trams had stopped.
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