Three people were killed, and several were seriously injured in a stabbing attack in a park in the southern English town of Reading on Saturday. 

Local Counter Terrorism Policing confirmed that the incident has been declared a terrorist incident.

Police said a 25-year-old man, who is thought to be Libyan, has been arrested at the scene, and counter-terrorism police will take over the investigation into the incident.

A video posted on Twitter showed paramedics rushing to help at least three people who were bleeding on the ground, footage which couldn't be immediately verified. 

Hours after the attack, about a dozen armed police officers carrying shields were seen entering a block of flats in Basingstoke Road in Reading, while other counter-terrorism police were also at the scene. 

The attack happened in Forbury Gardens at about 7 p.m. local time. Matt Rodda, a lawmaker who represents Reading in Britain's parliament, said the park, located in a historic area of the town, was busy on Saturday evening, in part due to current coronavirus restrictions that have forced venues like pubs to close, prompting people in Britain to gather in parks in the evenings to meet friends.

"This incident happened in that kind of environment, so it's really quite deeply shocking for local people," Rodda told Sky News.

A witness quoted by the Mirror newspaper said the knife-wielding man "shouted some unintelligible words" before stabbing people who had gathered in the park on a sunny summer evening in Reading, which is about 65 kilometers west of London.

Lawrence Wort, a 20-year-old personal trainer who said he had witnessed the attack, told the BBC that he was about 10 meters away when the attack started. He saw the lone man walk through a group of about eight to 10 friends before he suddenly began stabbing them after shouting some unintelligible words.

Wort said the assailant stabbed three of them and then locked eyes with him and the person he was with and started coming toward them, but they ran away to safety, at which point the man realized that he couldn't catch them and turned back to attack another group that was sitting down. When everyone started running from the scene, the attacker ran from the park, Wort said.

Detective Chief Superintendent Ian Hunter, from Thames Valley Police, said they are not treating the incident as a terrorist attack, but "officers are keeping an open mind as to the motivation for the incident and are being supported by colleagues from Counter Terrorism Policing South East."

Sources earlier told the BBC and Sky News that police were treating the incident as terrorism-related.

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

The head of the local council authority in Reading, Jason Brock, said his thoughts were "with the families of all those who have died or have been injured."

The attack in Reading took place at the site of a Black Lives Matter (BLM) anti-racism protest in the town earlier on Saturday, but police said it did not appear to be connected.

A BLM supporter said on Facebook the stabbings occurred a few hours after the protest had ended, and people who attended the demonstration were unharmed.

Some 36 people were killed in four attacks in Britain in 2017, which authorities labeled as terrorism, the most deadly of which occurred at the end of a concert by U.S. singer Ariana Grande in Manchester, northern England, with other attacks at the London Bridge and near parliament.

Last year the government downgraded the national terrorism threat level from "severe" to "substantial," meaning an attack is likely.

(With input from Reuters)