A Muslim man in his 70s was stabbed inside London’s main mosque just as Friday prayers were starting.

The assailant was arrested on suspicion of attempted murder, police said.

Images posted to social media showed a white man in a red hooded top, jeans and bare feet being pinned to the floor by police officers inside the mosque, as others, including a small child, watched on.

One video showed a knife on the floor under a plastic chair.

Prime Minister Boris Johnson tweeted: “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.”

The Metropolitan Police confirmed the injured man was in his 70s. He has been assessed in hospital and his condition is not life-threatening.

A crime scene has been put in place and inquiries continue.

The London Ambulance Service (LAS) said the victim had been taken to a major trauma centre.

The service tweeted: “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.”

A witness to the attack revealed the suspect had been a regular in the mosque.

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 has 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.”

Watik continued: “We were shocked, we didn’t know what happened, we saw the guy on the floor and blood on his shoulder and the knife on the floor. He was waiting for him I think to start praying. He was right behind him.”

When asked if there was any security at the mosque, he said no. “This is the mosque, you can never stop anybody, everyone comes to the mosque.”

The stabbing is not being treated as terror related, Scotland Yard said later. The mosque, the largest in the country, was built in 1977 with Saudi money and attracts worshippers from many countries.