A woman wearing a Charlie Hebdo T-shirt was attacked with a knife at Speakers’ Corner in London’s Hyde Park on Sunday, police said.
The 39-year-old woman was taken to hospital after the attack at the site where people gather for public speeches and debates.
Footage shared on social media shows someone dressed in black approaching a woman wearing a Charlie Hebdo T-shirt.
The woman is later seen clutching her right hand close to her body and with what appears to be blood at her temple as she is helped into a police van by officers who were nearby.
An attack at the offices of the French satirical magazine Charlie Hebdo by Islamic extremists killed 12 people in January 2015.
The magazine had published a cartoon of the Prophet Mohammed.
The Metropolitan Police, who described it as "a very distressing incident" for the victim, said officers were called on Sunday at just after 3.30pm to reports that a woman had been assaulted.
A statement said the woman had a minor slash injury to her head and was treated by the London Ambulance Service before being taken to a central London hospital.
Her injuries are not life-threatening, police said.
A knife was recovered nearby.
Officers based in Westminster are working with the specialist Metropolitan Police unit which patrols the royal parks to investigate and to try and find the person responsible.
No arrests have been made.
Detective Superintendent Alex Bingley, of the Central West Command Unit which covers policing in Westminster, said: "This was clearly a very distressing incident for the woman involved and officers have spent time with her, whilst she was being treated for her injury, to get an account of what happened.
"We know that this assault was witnessed by a number of people, many of whom captured it on their phones.
"I would ask them, if they have not already done so, to contact police.
"We remain in the early stages of our investigation and are working hard to trace the person responsible.
"I would ask people not to speculate on the motive for the attack until we have established the full facts."