Police were called to a mosque in London at 3:10pm following reports of a stabbing.
In a statement, the Metropolitan Police said that a man in his seventies was stabbed, but his condition was not believed to be life-threatening.
Footage of the aftermath of the attack was shared widely on social media.
The attack took place during Asr prayer — the third of the day — at London Central Mosque in Regent's Park, north London.
The victim was treated at the scene by paramedics before being taken to the hospital.
A man was arrested at the scene on suspicion of attempted murder. Police have said they are not treating the incident as terror related at this time.
Chief Superintendent Helen Harper said: "This was a profoundly shocking incident inside a place of worship and an urgent investigation is being carried out into the circumstances. Although we believe this to be an isolated incident, patrols have been increased around the area of the mosque to provide reassurance to worshippers and the local community.
"The investigation into this incident is being led by officers on Central-West borough command unit. A 29-year-old man was arrested at the scene and has been taken into custody. The man is believed to have been attending prayers inside the mosque.
"I would like to thank everybody in the mosque who helped to detain the man before he was arrested and continue to help my officers with their enquiries. I'm extremely relieved the the injuries suffered by the victim do not appear to be life-threatening. However, this incident has undoubtedly caused a great deal of concern and we are working as swiftly as possible to establish the circumstances. No other suspects are being sought at this time."
A London Ambulance Service spokesperson told BuzzFeed News: “We sent an ambulance crew, a paramedic in a car and an advanced paramedic practitioner to the scene.
“We treated a man for injuries and took him as a priority to a major trauma centre."
The London Central Mosque released the following statement on their website after the attack: "There was an incident today at the London Central Mosque where an unknown individual attacked and stabbed the Muazzin (the person who makes the call to prayer) during Asr Prayer around 3pm.
"The attacker was apprehended by the worshippers until the police arrived and arrested him.
"The Muazzin did not sustain any life threatening injuries but was seriously injured and is being treated at the hospital. We will provide further updates as we await information from the police regarding the incident, our thoughts and prayers are with the Muazzin and his family."
Prime Minister Boris Johnson tweeted he was "deeply saddened" by the attack and that his "thoughts are with the victim and all those affected."
Sadiq Khan, Mayor of London, tweeted that the Met police are "providing extra resources in the area" following the attack.
He added: "Every Londoner is entitled to feel safe in their place of worship & I want to reassure London's communities that acts of violence in our city will not be tolerated."