Counter-terrorism police in England are investigating after five people were stabbed at a shopping centre by a man who was "lunging at multiple people".
A man in his 40s has been arrested on suspicion of serious assault, Greater Manchester Police said.
Footage from the scene in Manchester city centre showed an officer pointing a taser at a man lying on his front on the ground, with a second officer on top of him.
The local ambulance service said one patient had suffered "serious" injuries.
In a statement, police said: "Specialist officers are continuing to respond to an incident at the Arndale Shopping Centre in the city.
"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."
Specialist officers are continuing to respond to an incident at the Arndale shopping centre in #Manchester city centre. pic.twitter.com/HCiyKaIBV3
— Greater Manchester Police (@gmpolice) October 11, 2019
British Prime Minister Boris Johnson tweeted: "Shocked by the incident in Manchester and my thoughts are with the injured and all those affected.
"Thank you to our excellent emergency services who responded and who are now investigating what happened."
Shocked by the incident in Manchester and my thoughts are with the injured and all those affected. Thank you to our excellent emergency services who responded and who are now investigating what happened.
— Boris Johnson (@BorisJohnson) October 11, 2019
A 23-year-old shop worker, who gave his name only as Jordan, described a man "running around with a knife lunging at multiple people" - and said someone had come into the store "visibly shaken with a small graze".
He added: "Soon after, security staff told all retail staff to close their doors and move the public to the back of the stores."
Footage also showed police officers running through the centre and several emergency services vehicles parked outside.
Witnesses say that people were evacuated from the shopping centre.
Freddie Houlder was in the 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 in to tears.
"I definitely don't think it was gang violence because this guy was going around trying to stab random people, I believe," he added.