Lawrence Wort, a personal trainer who witnessed the attack, said the park was crowded when one person suddenly "shouted some unintelligible words and went around a large group of around 10, trying to stab them".
A Black Lives Matter protest took place at the park earlier but police said there was no indication the two incidents were connected.
The man held by police on suspicion of killing three people in Reading is Khairi Saadallah.
Citing security sources, the BBC reported that Khairi Saadallah, 25, who was arrested at the scene on Saturday had came to the attention of MI5 past year after a tip-off that he had plans to travel to Syria - potentially for terrorism.
"The thoughts of everyone at Thames Valley Police are with those who have been affected by this tragic incident and I understand the concern that this will have caused amongst the community".
There is now a police cordon in place at Forbury Gardens, with people being urged to avoid the area while officers are on scene.
Police have also urged the public to stop sharing footage and photos from the crime scene as they battle to contain evidence.
Prime Minister Boris Johnson said he was "appalled and sickened" by the attack and said "we will not hesitate to take action" if there are lessons to be learned from the circumstances.
This is a developing story.
Overnight, heavily armed officers raided an apartment about a mile away from the park, where the suspect reportedly was living.
- The source added the man who was arrested is Libyan.
It states: "A freelance journalist who lives near the scene has said surrounding roads as well as a retail park have been cordoned off by police officers".
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.
The poster advertising these BLM protests said it was to be a "peaceful protest" that was to start at 1pm with a march, then followed at 2pm by "voices of the community" and at 4pm there was to be a "silence for victims unjustly killed".
In a social media post, she said she was "praying for the people that are affected".
The force added that officers and other emergency services attended the scene at around 7pm.
Lucy D'Orsi, Deputy Assistant Commissioner for the Met, told the PA news agency last week: "As we come out of lockdown, I think it is really important to remind people that the threat level from terrorism remains at substantial, which means an attack is likely".
"We didn't know if it was a situation like happened in London a few years ago, where the attackers were on the loose, " he said.
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