British police said Saturday "a number of people" had been injured in a crowded park in the southern English city of Reading, in what witnesses called a multiple stabbing attack.

Sources told Sky News and Britain's Press Association that early indications suggested the incident was terror-related.

Prime Minister Boris Johnson immediately condemned the "appalling incident" at the Forbury Gardens park in the historic centre of the city of around 220,000.

"A number of people sustained injuries and were taken to hospital for treatment," the Thames Valley Police said in statement.

The police reported arresting "a man at the scene" and urged everyone to stay away.

The Press Association said the arrested man was Libyan.

"My thoughts are with all of those affected by the appalling incident in Reading and my thanks to the emergency services on the scene," Johnson tweeted.

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

"In terms of the protest and the people who attended from Black Lives Matter, we're all safe," the Reading BLM event organiser Nieema Hassan said on Facebook.

"None of us are affected. We had all left by the time this happened."

Police have sealed off the scene of a 'serious incident' in the southern English city of Reading
Police have sealed off the scene of a 'serious incident' in the southern English city of Reading AFP / Adrian DENNIS

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

One witnessed 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," witness 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".

The assailant that then lunged at another group and "got one person in the back of the neck," Wort said.

"Then when he realised everyone was starting to run, he ran out the park," said the witness.

Officials said two people were being treated in the emergency department of Reading's Berkshire Hospital.

"An horrific, dreadful incident," policing minister Kit Malthouse tweeted.

The main opposition Labour party's leader Sir Keir Starmer called in the incident "very concerning".