An attacker stabbed a senior figure at a London mosque during afternoon prayers on Thursday.
The city's Metropolitan Police force said detectives investigating the stabbing do not think it was terror-related. Police did not speculate on a possible motive.
The 29-year-old suspect attacked the muezzin — summons the faithful to pray — at London Central Mosque in Regent's Park.
He was apprehended by worshippers, according to the mosque, before police arrived to arrest him on suspicion of attempted murder.
The muezzin — a man in his 70s — was treated by paramedics before being taken to hospital. Police said his condition is not thought to be life-threatening.
"There was an incident today at London Central Mosque where an unknown individual attacked and stabbed the muezzin during Asr Prayer around 3 pm," said the London Central Mosque in a statement on its website.
"The attacker was apprehended by the worshippers until the police arrived and arrested him.
"The muezzin did not sustain any life-threatening injuries but was seriously injured and is being treated at the hospital.
"Our thoughts and prayers are with the muezzin and his family."
Witnesses say attacker attended mosque before
One witness, Abi Watik said the arrested man had attended the mosque for several months. He said the suspect stabbed the victim in the shoulder just as afternoon prayers began.
''He was praying behind him and then he stabbed him," Watik said.
''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 said.
Ayaz Ahmad, an adviser to the mosque, said the suspect was "not a regular" but had been noticed around the mosque before.
UK leaders react
London Mayor Sadiq Khan said on Twitter that he was deeply concerned by this incident at London Central Mosque.
Prime Minister Boris Johnson tweeted that he was ''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,'' Johnson wrote. ``My thoughts are with the victim and all those affected.''