The multiple stabbing in Reading is suspected to be terror-related, a security source has told the PA news agency.
Multiple people have been rushed to hospital following the incident at Forbury Gardens, a park in the town centre, at around 7pm on Saturday.
Reports suggest three people have died, although this has not been confirmed by police.
Amy Lewis and Rohit Kachroo provide update on Reading stabbings
A Libyan man has been arrested in connection to the incident, ITV News Security Editor Rohit Kachroo has learned.
He added police have yet to formally declare the incident terror-related, although a terror link is suspected. Police are yet to carry out a mental health assessment so a motive has yet to be established.
Armed police are now carrying out a search at a block of flats in Reading following the stabbings.
In a statement, Thames Valley Police said: "Officers arrested a man at the scene who is now in police custody.
"A number of people sustained injuries and were taken to hospital for treatment.
"There is a police cordon in place at Forbury Gardens and we would ask that the public avoid the area while officers are on scene.
"We would ask members of the public not to share images or videos of the incident on social media, but instead to report these to the police.
"We would ask that anyone with information relating to this incident contacts Thames Valley Police by calling 999, quoting reference 1159 20/6."
A Black Lives Matter protest had taken place in the park earlier in the day although there is no suggestion the two events are linked, Thames Valley Police said.
Prime Minister Boris 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."
Home Secretary Priti Patel tweeted: “Deeply concerned to hear reports of an incident in Reading.
“My thoughts are with everyone involved, including police and emergency responders at the scene.”
The leader of Reading Council Jason Brock tweeted: “Concerning reports from Reading town centre – please stay clear of the area as Police are dealing with a serious incident.”
Freelance journalist Claire Gould, 33, who lives near the scene, told the PA news agency she saw the 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.
There was a heavy police presence near the scene and two air ambulances were dispatched to the scene.
A cordon remains in place around the scene and investigations remain ongoing.