READING: British police said on Sunday they were treating a stabbing spree in which a lone assailant killed three people in a park filled with families and friends in the southern English city of Reading as a `terrorism incident` The Thames Valley police said three people were also seriously injured and a 25-year-old resident of the historic town 60 kilometres west of London was detained.

Local and counterterrorism police made no reference to reports that the man was a Libyan refugee who has been granted asylum in Britain.

Footage showed what appeared to be a large number of counterterrorism police performing a controlled explosion at the suspect`s presumed residence shortly after the Saturday evening attack.

`This morning, I can formally confirm that this has been declared a terrorist incident,` counterterrorism police chief Neil Basu said.

`Although the motivation for this heinous act is far from certain, Counter Terrorism Policing South East has taken on responsibility for leading this investigation.

Prime Minister Boris Johnson said he was `appalled and sickened` by the bloodshed at Forbury Gardens a picturesque park situated next to the ruins of the 12th-century Reading Abbey.

`If there are lessons that we need to learn about how we handle such cases, about how we handle events leading up to such cases, then we will learn those lessons, and we will not hesitate to take action when necessary,` Johnson said in televised comments.

A Black Lives Matter protest took place at the park earlier but police said there was no indication the two were linked.

`None of us are affected, the Reading BLM event organiser Nieema Hassan said on Facebook. `We had all left by the time this happened.

Britain`s terror alert level was kept at `substantial`the third-highest on a five point scale.

`There is no specific intelligence to suggest anyone else attending crowded places is at risk,` the counterterrorism police chief said.

Witnesses reported seeing two air ambulances and several police cars rush to the park on Saturday evening.

One witness described a lone assailant walking through a park filled with people relaxing on the grass and stabbing them at random.

`The park was pretty full. A lot of people sat around drinking with friends,` Lawrence Wort told the Press Association.

`One lone person walked though, suddenly shouted some unintelligible words and went around a large group of around 10, trying to stab them.` Wort said he saw three people being stabbed `in the neck and under the arms`.-AFP