(CNN)Five people were injured Friday in a stabbing at a shopping mall in Manchester, northwest England, and a man is being detained under the UK's Mental Health Act, police said.
The incident took place at the Arndale Centre at around 11:15 a.m. local time (6:15 a.m. ET), where a man who was carrying a "large knife" began "lunging and attacking people," Assistant Chief Constable Russ Jackson of Greater Manchester Police said.
Jackson said while the injuries are "nasty," none of them are life-threatening.
He said police did not know the motivation behind the attack, adding that "it appears random, and it is brutal."
Following the attack, a man originally was arrested on suspicion, preparation and instigation of an act of terrorism. He is believed to have acted alone.
A statement tweeted later Friday by Greater Manchester Police said: "The 40-year-old man, who was arrested on suspicion of committing a terrorism offence, has now been assessed by specialist doctors and compulsorily detained under the Mental Health Act. The investigation will continue in order to fully understand the motives behind the attack."
Jackson said the man was within police custody within five minutes.
Witness Freddie Houlder, 22, who was shopping for shoes at the time, told CNN that he "heard screams" before a woman ran into the shop and "said someone tried to stab her."
"She said this guy was trying to stab random people," Houlder said. "We were locked in the shop for around 20 minutes, then were finally allowed out."
John Worsley, 25, was also shopping at the time and told CNN that after he heard screaming he "walked round the corner to find a woman on the floor next to the cash machines."
"A lady was around her screaming she had been stabbed," Worsley told CNN. "We then saw the attacker running out of the doors."
Among those injured was a 19-year-old woman who was taken to hospital with stab wounds, according to police. Another woman and a 50-year-old man were also hospitalized after sustaining stab injuries.
Prime Minister Boris Johnson called the attack a "horrific incident," and that he was "very grateful to the emergency services and to the police."
A spokesperson for North West Ambulance service told CNN earlier that they had received "multiple" calls for help at 11:17 a.m. local time and that several people were being treated by emergency services.
In a statement posted online, the service said it had "dispatched 13 ambulances, four hazardous area response vehicles, two advanced paramedics, two doctors, and two officers to the scene."
The spokesperson added that a "number of resources" -- one ambulance, two hazardous response vehicles and a physician -- had been deployed to the mall.
A large number of officers were pictured at the scene via social media posts.
No one else is believed to be involved in the attack, police said, but added that they will "keep this under review."
According to Britain's PA news agency, the mall remains closed and a police cordon is in place.
The mall is just a short walk from Manchester Arena, where 22 people were killed following a suicide bombing outside an Ariana Grande concert.
Also nearby is Victoria railway station, where three people were stabbed on New Year's Eve.