© Claire Gould/TwitterSeveral people are in hospital with two in a critical condition, after the atrocities in Forbury Gardens, Reading on Saturday night.
A 25-year-old man from Reading, about 40 miles west of London, was arrested at the scene on suspicion of murder and is in custody, Thames Valley police said. They are not looking for anyone else in connection with the incident.
The attack was "not currently being treated as a terrorism incident". However, DCI Ian Hunter said police were keeping an open mind on the motive and were being supported by counter-terrorism police.
Hunter said: "There is no intelligence to suggest that there is any further danger to the public. However, we urge people to remain vigilant and report anything suspicious by calling police."
Several hours after the stabbings, police raided a block of flats in the town. Photos from the scene of the raid showed counter-terrorism specialist firearms officers were present.
Hunter said there would be an increased police presence in the area amid continuing inquiries. "I would urge the public to avoid the area at this time and thank the community for their patience. We will let you know as soon as the area is back open," he said.
Reacting to the incident on Sunday morning, the health secretary, Matt Hancock, told Sophy Ridge on SkyNews: "The police are clearly investigating. They have not currently stated that this is a terrorism investigation but rather a murder investigation."
He added: "My heart goes out to the families, the friends of the victims - clearly it's a shocking event. The police did a brilliant job last night, firstly during the event and then afterwards. It's now an operational matter, and we should give them all the space and the support that we need to get to the bottom of it."
Officers and other emergency services were called to reports of "multiple stabbings" at a park in Forbury Gardens, Reading, at 7pm on Saturday . The suspect, who is understood to be Libyan, was detained at the scene.
Police earlier said several casualties were taken to the Royal Berkshire hospital.
Disturbing videos on social media showed scenes of victims receiving resuscitation treatment.
Shortly after the stabbings, initial inquiries led to police calling in counter-terrorism investigators, with the security service, MI5, assisting the investigation.
Senior government officials including Boris Johnson, the prime minister, and Priti Patel, the home secretary, are being kept informed of developments.
Suggestions it was linked to a Black Lives Matter protest that took place in the park earlier on Saturday were dismissed by police. Organisers said the peaceful protest ended nearly two hours before the incident. Crowds who attended the rally had dispersed without any trouble, a spokeswoman said.
Two air ambulances and several police vehicles responded to the incident.
Johnson tweeted: "My thoughts are with all of those affected by the appalling incident in Reading and my thanks to the emergency services on the scene."
Patel said that she was "deeply concerned" by the developments.
Laurence Wort, a witness, told the BBC the park was full of people sitting in groups when a lone person walked through, shouted something and started attacking a large group.
"He stabbed three of them and then turned and started running towards me, when we turned and started running. When he realised that he couldn't catch us he tried to stab another group sat down," said the 20-year-old, who was there on a day trip. "When he realised everyone was starting to run, he ran out the park."
Claire Gould, a freelance journalist who lives near the scene, told PA Media she saw emergency air ambulance vehicles land on the nearby Kings Meadow as a police helicopter circled overhead.
She described hearing lots of sirens and said surrounding roads and a retail park had been cordoned off by officers.
The leader of Reading borough council, Jason Brock, tweeted that he was "shocked and appalled" following the attack.
"My immediate thoughts are with the families of all those who have died or have been injured," he said. "All of us at Reading Council condemn this horrific and senseless attack in the strongest possible terms. We also offer our continued support to the emergency services who responded so quickly."
The National Police Chiefs Council (NPCC) urged people to avoid speculating about the attack or sharing video or images of it.
Keir Starmer, the Labour leader, described the stabbing as "very concerning" and urged people to listen to the NPCC and not share footage of the incident that has been circulated online.
In November, the UK's national terrorism threat level was lowered from severe to substantial for the first time in five years. The "substantial" threat level means there is a high level of threat and an attack might well occur without further warning.
Since the UK terrorism threat was lowered, there have been two jihadist attacks. Weeks after the official announcement, on 29 November, Jack Merritt, 25, and Saskia Jones, 23, were stabbed to death by Usman Khan, 28, who was tackled by people at a prisoner rehabilitation conference at London Bridge. Khan's attack was halted only when he was shot dead by armed officers.
On 2 February, Sudesh Amman, who had been recently released while serving a terrorism sentence, stabbed two people in Streatham, south London. He wore a fake suicide vest and armed surveillance officers had been following him and shot him dead.
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