Scotland Yard has launched an investigation into the attempted murder of a man who was stabbed at a mosque during afternoon prayers before his alleged attacker was wrestled to the ground by worshippers.

The mosque’s muazzin, who calls others to prayer, was stabbed in the neck in front of hundreds of horrified onlookers at London Central Mosque in Regent’s Park, in what one witness described as “30 seconds of mayhem”.

Online footage showed the alleged perpetrator, a 29-year-old man, being pinned to the ground by police. While one witness claimed that he was a regular at the mosque, another said that he had never seen him before.

The Metropolitan police, who arrested the alleged attacker on suspicion of attempted murder, said they were not treating the incident as terror-related.

The victim, named locally as Raafat Maglad and said to be around 70 years old, has attended the mosque for 25 years. He was rushed to hospital after being treated by paramedics at the scene. He was “seriously injured”, the mosque said, but his condition was not life-threatening.

A Twitter user posted footage of a man wearing jeans and a red top, sitting cross-legged in the mosque, apparently being held in cuffs behind his back by a police officer. The user said the victim had been taken to hospital after being “stabbed in the neck” during prayers.

Facebook Twitter Pinterest The suspect being detained by a police officer. Photograph: Twitter

In a statement, the Met said: “A 29-year-old man, who is believed to have been attending prayers, was arrested inside the mosque on suspicion of attempted murder. He has been taken into custody at a central London police station. The incident is not being treated as terror-related at this time.”

One witness said the suspect had been seen at the mosque several times in recent months.

Abi Watik, 59, said: “We were shocked. We didn’t know what was happening.” He said the victim had been stabbed once in the right shoulder moments after prayers had started, and he believed the suspect had waited for that moment.

“It had just started and he was waiting for them to start. He was praying behind him and then he stabbed him. He [the suspect] was silent the whole time.”

Ayaz Ahmad, an adviser to the mosque, was doing the Asr afternoon prayer when the victim, who was standing behind the imam, was attacked. “It was during the first rakat of the prayer,” Ahmad said. “We all tussled with him [the suspect], a few hundred people were worshipping at the time. The muazzin has been here for 25 years, calling the prayer five times a day. He is very respected.

“The attacker is not known to us, a few have seen him before. I heard the scuffle, the muazzin yelled and screamed, the imam ended the prayer. It was 30 seconds of mayhem. Everybody’s reaction was shock and horror, the men were screaming.

“It was a vicious attack. We live in a society where we hear a lot about knife crime and what have you, but actually to physically be in a situation where you’re first-hand watching an attempted murder take place, it’s very shocking.”

He added: “This guy was not a regular. I’ve never seen him here in 10 years.”

Waleed Mohammed, a friend of the victim, described him as a well-loved community figure who did not know his attacker. He said: “He was a witness at my wedding, he is a gentle person, so welcoming, well loved in the community. He didn’t know the perpetrator.”

Adnan Idha, 54, a regular worshipper at the mosque, said the attacker had spent time with his victim and had been seen sitting with him in his office. He told PA the attacker had joined the mosque around three months ago, and he had seen him in recent days at evening prayer.

Around 6,000 people a week worship at the mosque, which is one of the largest Islamic places of worship in the UK.

Boris Johnson tweeted that he was “deeply saddened” by the attack and added: “It’s so awful that this should happen, especially in a place of worship.”

Boris Johnson (@BorisJohnson) I'm deeply saddened to hear of the attack at the London Central Mosque. It's so awful that this should happen, especially in a place of worship. My thoughts are with the victim and all those affected.

Images posted on social media showed the alleged attacker, who was barefoot, being pinned to the floor of the mosque by police officers as worshippers looked on. In one video, a knife was seen on the floor under a plastic chair.

A statement on the mosque’s website confirmed that the victim had been calling worshippers to prayer. “There was an incident today at 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,” a statement said.

“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 await further information from the police regarding the incident. Our thoughts and prayers are with the muazzin and his family.”

Helen Harper, chief superintendent for the Met’s central-west borough command unit, said although it was believed to be an isolated incident, patrols had been increased around the area of the mosque to provide reassurance to worshippers and the local community.

She added: “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 inquiries. I’m extremely relieved 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.”

The ambulance service said: “We sent an ambulance crew, a paramedic in a car and an advanced paramedic practitioner to this incident in Regent’s Park. We treated a man at the scene and took him to a major trauma centre.”