British police are treating a stabbing rampage in the southern English town of Reading, in which three people were killed and three others seriously wounded, as terrorism.

A 25-year-old man was arrested on suspicion of murder after the stabbings on Saturday evening in a park in Reading, about 65 kilometres west of London.

A security source told Reuters that the man, who remains in police custody, is a Libyan.

A man, initially identified in media reports as a Libyan asylum seeker known as Ahmed Hannachi, was subdued at the scene and was in police custody.

“Counter Terrorism Policing can now confirm that the stabbing incident that happened in Reading last night has now been declared a terrorist incident,” Thames Valley Police said in a statement.

“Deputy Assistant Commissioner Dean Haydon, Senior National Co-ordinator for the Counter Terrorism Policing [South East] network, has this morning declared the incident a terrorist incident, and CTPSE will be taking over the investigation.”

🚨| UPDATE- Murder investigation Reading@TerrorismPolice can now confirm that the stabbing incident that happened in Reading last night has now been declared a terrorist incident.

We continue to work together with Counter Terrorism Policing SE.

➡ https://t.co/CMkzwE6crv pic.twitter.com/MQeQr8XLe5

— Thames Valley Police (@ThamesVP) June 21, 2020

Prime Minister Boris Johnson, who has described the incident as appalling, held a meeting on Sunday with security officials, senior ministers and police to be updated about the investigation, a spokesman said.

Initially police and the government had said the incident did not appear to be terrorism and they were keeping an open mind as to the motive.

They said they were not looking for any more suspects in relation to the incident.

The stabbings took place on a sunny evening hours after a rally by anti-racism protesters in the park, Forbury Gardens, but appeared to be unrelated to that.

“The Black Lives Matter event had finished some three hours previous and was by all accounts a very well-conducted peaceful demonstration and this is not a connected attack with that whatsoever,” Jason Brock, the head of the local council authority, told the BBC.

Flowers and a message left at the Abbey gateway of Forbury Gardens in Reading. Photo: PA

Current coronavirus restrictions mean venues like pubs are closed, so many people in Britain gather in parks in the evenings to meet friends.

“Incidents of this nature are very rare, though I know that will be of little comfort to those involved and understand the concern that this incident will have caused amongst our local community,” Chief Constable John Campbell of Thames Valley Police said.

“We would like to hear from anyone who has video footage. Out of respect for those deceased and injured, along with their loved ones, please do not circulate this footage on social media – this will be incredibly distressing.”

A witness said the attack began when a man suddenly veered toward a group of about eight to 10 friends and began stabbing them.

The nature of the attack was reminiscent of a number of recent incidents in Britain that authorities considered to be terrorism.

In February, police shot dead a man, previously jailed for promoting violent Islamist material, who had stabbed two people on a busy street in south London.

Last November another man who had been jailed for terrorism offences stabbed two people to death on London Bridge before he too was shot dead by police.

-with AAP