Four people have been injured in a knife attack at the Arndale Centre in Manchester city centre.
A man was seen « running around with a knife lunging at multiple people », according to one witness.
Police said three people had been stabbed and a fourth was also hurt. A man in his 40s has been arrested.
Prime Minister Boris Johnson tweeted: « Shocked by the incident in Manchester and my thoughts are with the injured and all those affected. »
Greater Manchester Police (GMP) said counter-terror officers were « keeping an open mind ».
The arrested man has been held on suspicion of serious assault.
Two women, including a 19-year-old, are in a stable condition in hospital, while a man in his 50s is also being treated in hospital for stab wounds, police said.
North West Ambulance Service said one patient had suffered « serious » injuries.
- Live updates: Manchester Arndale stabbings
The shopping centre has been evacuated and trams have stopped operating at Exchange Square outside the entrance.
A spokesperson for the force said: « 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. »
Social media posts have shown a large number of officers at the scene and the suspect being Tasered.
An image sent to the Manchester Evening News appears to show a patient being treated at a Starbucks cafe in the centre.
David Allinson, centre director of Manchester Arndale, said: « The centre has been evacuated while police investigate the incident.
« We are unable to comment any further as the incident is currently being handled by Greater Manchester Police, who we will continue to support with their investigation. »
At the scene
By Kaleigh Watterson, BBC News Online
There’s a helicopter hovering up above and various police cars stationed around, with a large cordon set up close to Selfridges.
This should be a busy Friday afternoon of shopping in Manchester, and there’s an eerie atmosphere as the city centre’s largest mall is deserted.
People are gathered around watching the police activity, while others seem to be carrying on with their day.
People who were near the shopping centre at the time have been describing what they saw.
Deputy Mayor of Greater Manchester Bev Hughes praised the emergency services.
Labour councillor Pat Karney, for Harpurhey and Collyhurst, tweeted: « Armed police on guard. Shocking scenes right out of a movie but real people with injuries. »
A shop worker, who gave his name only as Jordan, 23, said: « 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.
Freddie Holder, 22, from Market Drayton, Shropshire, said 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 ».
« 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 added.
« I’m still kind of in shock from it, I’m shaking a little bit… all shops had been locked down just for safety.
« The police arrived extremely quickly, which was very lucky. »
Councillor Nigel Murphy, deputy leader of Manchester City Council, said the « incident is under control » and advised people to stay away from the scene to enable the emergence services to carry out their work.
« We would encourage people not to speculate about this incident but to refer to trusted official sources for updates, » he added.
Bev Hughes, Greater Manchester’s deputy mayor for policing, praised the emergency services’ response, adding: « The scene has been secured and all emergency services are engaged in ensuring public safety – the investigation is ongoing. »
GMP tweeted: « A rendezvous point has been established at Piccadilly Gardens, so you will see a large police presence in this area. Please don’t be alarmed. »
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