The Prime Minister has said he was "appalled and sickened" after police declared the murder of three people in a Reading park to be a terrorist attack.
The incident, which took place at 19:00 on Saturday in Forbury Gardens in the town centre, also left another three people injured.
One of the victims has been named by The Holt School in Wokingham as James Furlong, its head of history and government and politics.
A 25-year-old man from Reading was arrested at the scene on suspicion of murder and is currently in custody.
The suspect is thought to be an asylum seeker from Libya.
Prime Minister Boris Johnson said he is "appalled and sickened" by the suspected terror attack and pledged "we will not hesitate to take action" if there are lessons to be learned.
Thames Valley Police said on Sunday morning the attack is now being treated as terrorism and that counter-terrorism police would now be taking over the investigation.
Chief Constable John Campbell told reporters the first emergency call was made to police at 18:56 on Saturday and that a man was arrested within five minutes.
He added there is not believed to be a wider risk to the public and there is nothing to suggest anyone else was involved in the attack at Forbury Gardens in Reading.
He said on Sunday: "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.
"We will be working closely with our partners over the coming days and weeks to support the Reading community during this time, as well as with Counter-Terror Police South East (CTPSE) as they progress their investigation."
He asked the public to avoid the area and also to "avoid speculation" about the attack.
Police have urged social media users to not post footage of the attack out of respect for the victims.
Footage of the aftermath of the attack in which the victims were receiving first aid appeared online on Saturday evening.
The Prime Minister has also held a meeting with security officials, police and senior ministers over the incident.
In an interview in Downing Street, Boris Johnson said he is "appalled and sickened that people should lose their lives in this way and our thoughts are very much with the family and the friends of the victims today".
"I’ve obviously talked to the chief constable at Thames Valley Police, thanked him and his officers for their bravery in tackling the suspect.
"We now have someone in custody. The police must get on with their job, get to the bottom of exactly what happened, and so it would be difficult really to comment in detail.
"Except to say this, if there are lessons we need to learn about how we handle such cases, how we handle the events leading up to such cases, then we will learn those lessons and we will not hesitate to take action where necessary," Mr Johnson said.
One of the patients admitted to Royal Berkshire Hospital has been discharged, while another remains in a stable condition under observation, a spokeswoman said.
The head of counter terrorism policing, Metropolitan Police Assistant Commissioner Neil Basu, said there is nothing to suggest anyone else was involved in the attack and police are not currently seeking anyone else as part of the investigation.
Mr Basu said counter terror police would be working "around the clock", and thanked 41 witnesses who had come forward to assist police with their inquiries.
He also paid tribute to the "bravery" of police officers and members of the public at the scene in Reading.
Labour leader Sir Keir Starmer said this is "not a time for party politics" and that he is willing to work with the Government to see if there are "lessons that can be learned" following the Reading terror attack.
He said: "It’s horrific that it happened at all wherever it happened, and across Reading, across the country there will be communities really upset and worried about this, united in their grief.
"And all of our thoughts are with those who have lost someone in this."
"It’s very important that the investigation runs its course but I will want to work with the Government in response to this, to look at whether there’s lessons that can be learned, whether there need to be changes to the law," Mr Starmer added.
"But I think as a constructive opposition, I will want to speak to the Prime Minister to discuss how together, we can improve the response, learn from this.
"The investigation must be completed, but of course, this is not a time for party politics."
Cover image: Police at the Abbey gateway of Forbury Gardens in Reading town centre on Sunday morning (Picture: PA).