The 'knifeman' who stabbed a mother and her 11-year-old daughter in Leicester Square did not know them and was a stranger, police believe.
The Metropolitan Policesaid a man, 32, thought to be the only suspect, had been arrested after the attack in the tourist hotspot in broad daylight on Monday.
Scotland Yard detectives currently do not believe the man knew the family and are trying to establish his motive and what happened.
At this stage, the force says there was nothing to indicate it was terror-related.
Detective Chief Superintendent Christina Jessah described the incident as 'horrific' and paid tribute to the members of the public and nearby staff who 'bravely' stepped in.
A shop security guard, who gave his name as Abdullah, 29, and works at TWG Tea shop in the square, was one of those who tried to save the schoolgirl as she was being attacked today.
This is the moment a 'knifeman' was arrested after a schoolgirl and her mother were attacked in Leicester Square
The guard gave his name as Abdullah, 29, (pictured) and works at TWG Tea shop in the square
Forensic officers in blue overalls were seen examining the crime scene at the TWG tea shop
The Metropolitan Police said a man, thought to be the only suspect, had been arrested after the attack in the tourist hotspot on Monday
The guard said:'I heard a scream, at that moment I saw there was one person, roughly [in their] mid-30s or early 30s, and he was like stabbing a kid'
'I jumped on him, held the hand in which he was [carrying] a knife, and just put him down on the floor and just held him and took the knife away from him'
Met Police officers stand guard outside the cordon toprevent members of public breaching the crime scene
He said: 'I heard a scream, at that moment I saw there was one person, roughly [in their] mid-30s or early 30s, and he was like stabbing a kid.
'I jumped on him, held the hand in which he was [carrying] a knife, and just put him down on the floor and just held him and took the knife away from him.
'Then a couple of more people joined as well, and we just held him until the police came, it took maybe three to four minutes for the police to arrive and then they just took him into custod
Police said two victims, the girl and her 34-year-old mother, had been taken to a major trauma centre following the incident.
The girl was seriously injured and will require hospital treatment but her condition is not life threatening, while her mother suffered more minor injuries, police said.
The area around TWG Tea remained cordoned off throughout the afternoon while three police officers prevented members of public breaching the crime scene.
Two forensic officers in blue overalls arrived later and were seen examining a bloody piece of paper, a black baseball cap, a car key and two lighters which were left on the spot where the attack happened.
One of the crime scene examiners later moved inside the tea stockists where several green cardboard boxes lay strewn across the floor.
Abdullah said he and his colleagues gave first aid to the child before the police took over and that he did not know if the girl was with family members at the time.
'I just saw a kid getting stabbed and I just tried to save her. It's my duty to just save them,' he said.
The area outside a tea shop next to the LEGO story had been cordoned off with police tape
A police cordon has been put in place surrounding the double doors of the TWG Tea shop on Leicester Square
DCSI Jessah said those who had put themselves at risk to save them 'showed the best of London in doing so'.
'At this stage we don't believe the suspect and the victims were known to each other,' she said.
'While we continue to work to establish the suspect's motive, at this stage there is nothing to indicate the attack was terror-related.'
The square and the surrounding area attracts an estimated 2.5 million visitors every week and are home to shops, theatres, cinemas and restaurants.
A police cordon sealing off the crime scene surrounded the double doors of the TWG Tea shop, which is beside the Lego store and in front of the M&Ms store.
In a statement, police said: 'Officers are at the scene of a stabbing in Leicester Square.
'A man has been arrested and is in custody.
'We don't believe there are any outstanding suspects.
'Two victims, an 11-year-old girl and a 34-year-old woman, have been taken to hospital and we await an update on their condition.'
There were no immediate details about how badly injured they were. Police say a man has been arrested and is currently being held in custody
A mother and her child were treated at the scene by paramedics from London Ambulance Service
Detective Chief Superintendent Christina Jessah, in charge of policing for Westminster, said: 'This is a horrific incident and our thoughts are with the victims and their family. We will continue to provide support to them over the coming days and weeks.
'I would like to pay tribute to the members of the public, including staff from local businesses, who bravely intervened in this incident. They put themselves at risk and showed the best of London in doing so.
'An urgent investigation is now ongoing and detectives are working to establish the details around exactly what happened.
'At this stage we don't believe the suspect and the victims were known to each other.
'While we continue to work to establish the suspect's motive, at this stage there is nothing to indicate the attack was terror-related.
'Detectives will be keen to speak to anyone who was in the area and may have information that can assist with their enquiries. I would urge anyone who can help to get in touch.'
Witnesses, or anyone with footage or photos of the incident, are asked to call 101 or to contact Crimestoppers, anonymously, on 0800 555 111, giving the reference 2745/12AUG.
A London Ambulance Service spokesmansaid: 'We were called at 11.36am on Monday August 12 to reports of a stabbing at Leicester Square.
'We sent resources to the scene, including an ambulance crew, an advanced paramedic and an incident response officer. We also dispatched members of our tactical response unit.
'We treated a child and an adult at the scene and took them to a major trauma centre.'
A witness said the suspect in the attack in Leicester Square was a young, white, skinny male wearing a black T-shirt and jeans.
The witness,who gave his name as Desmond, 45, andperforms as Darth Vader on the square 'every day', told reporters: 'When I looked to my side I saw a young man... was stabbing her several [times] with a knife.
'Before I could jump down to go [over there], two guys were just passing, and they grab him, they put him on the ground, and immediately the police came. I think the woman was also stabbed once, but the child was the most targeted, he was stabbing the child several times. Police held him and stood him up and cuffed him.'
The cordon is beside the LEGO store and in front of the M&M store, and families and tourists are still standing in queues for each
A baseball cap at the scene in Leicester Square, London, as a man has been arrested after an 11-year-old girl and 34-year-old woman were stabbed
Desmondadded: 'It was so terrible, I've never seen a thing like that. I was heartbroken, I saw the woman was screaming with all her strength.'
Another, David Hawkes, 43, from Billingham, Teesside, was on a family trip to London to visit the Harry Potter Studios when he saw armed cops bundle the suspect in to a police van.
He said: 'We were sat in Leicester Square and heard a female scream, we walked across at that point police were just arriving quite promptly followed by ambulance crew.
'There was a police helmet on the floor and a guy wearing black had his hands in cuffs and armed police were putting him in a van.
'An air ambulance was trying to land and it was circling overhead.
Desmond, 45, (pictured) who performs as Darth Vader on the square 'every day', told reporters: 'When I looked to my side I saw a young man... was stabbing her several [times] with a knife'
He added: 'Before I could jump down to go [over there], two guys were just passing, and they grab him, they put him on the ground, and immediately the police came'
'The area around Lego store was taped off and police and armed police went inside the TWG tearoom.'
A police cordon has been put in place surrounding the double doors of the TWG Tea shop on Leicester Square.
The cordon is beside the LEGO store and in front of the M&M store, and families and tourists are still standing in queues for each.
At least six police officers are at the scene,which is popular among tourists and shoppers.
A Met spokesman said: 'We don't believe there are any outstanding suspects.
'Two victims, an 11-yr-old girl and a 34-yr-old woman, have been taken to hospital and we await an update on their condition.'
A London Ambulance Service spokesman said: 'We were called at 11.36am on Monday August 12 to reports of a stabbing at Leicester Square.
'We sent resources to the scene, including an ambulance crew, an advanced paramedic and an incident response officer. We also dispatched members of our tactical response unit.
'We treated a child and an adult at the scene and took them to a major trauma centre.'
