The shopping centre has been evacuated and one man has been held down by police with tasers.
A spokeswoman for North West Ambulance Service said paramedics had treated four people. No details on their conditions were immediately available.
Footage posted on Twitter showed a police pointing a Taser at a man lying on the pavement outside the shopping centre.
Another is seen handcuffing the man.
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.
A reporter for the Manchester Evening News said he seen paramedics wheeling a stretcher into the centre and was told two people were “‘randomly stabbing people”.
Others say that people aren’t allowed to leave the centre and other shops have been evacuated:
More to follow
Images via Twitter