A mother who was nearby when a six-year-old boy was thrown from a 10th-floor viewing platform at the Tate Modern art gallery said the incident was “terrifying”.
The child is in a London hospital after being found on a fifth-floor roof at the world-famous attraction.
A Metropolitan Police spokesman told PA that the boy is “no longer in a life-threatening situation”, adding: “He is critical, but stable.”
A 17-year-old male suspect remains in custody on suspicion of attempted murder.
The teenager had remained with members of the public on the 10th-floor viewing platform after the incident on Sunday afternoon, police said.
They added that there was nothing to suggest that he is known to the victim.
Officers had been called to the gallery at around 2.40pm and the child was treated at the scene before being flown to hospital by London’s Air Ambulance.
Olga Malehevska was on the viewing platform with her four-year-old son when the incident took place and described what happened as “absolutely terrifying”.
The journalist, from Ukraine, said: “I was standing on the balcony with my little one and he was touching the fence and we were making pictures.”
She said she then heard a noise and there was some pushing. She pointed out that the platform was not overcrowded.
“I just felt like something is going on, I should take my child out of there immediately and we tried to go towards the exit,” she said.
Ms Malehevska said she could hear people say “Oh my God, the boy dropped”, and also saw a woman crying, shaking and shouting “oh my son, my son”.
She said they were all kept inside the building for around an hour and 20 minutes.
Ms Malehevska said she was amazed at how quickly the emergency services arrived. She said it would not be possible for a child to climb the fence on the platform or jump through it.
A small group of tourists gathered outside the entrance of the Tate Modern ahead of it opening its doors at 10am on Monday.
A number of them were aware of the incident on Sunday.
Andrea Cioci, from near Rimini in Italy, said: “I’m glad to come here to see the art but if I think about that a part of my heart is sad.”
American tourists Aleta Kennedy and her daughter had pre-booked tickets for an exhibition at the gallery.
Ms Kennedy said of her daughter: “For her, this is one of the highlights of her trip.”
Ylenia Trivigno, from Tuscany in Italy, said it is her first visit to the Tate, adding: “It’s very sad.”
A Tate spokeswoman said the gallery will be open as usual on Monday but the viewing platform will remain shut “out of respect”. A sign inside the building informed visitors: “The viewing level is closed today.”
A photocall for a new exhibition which had been due to take place on Monday morning was postponed.
In a short update, Scotland Yard said: “Officers continue to work hard to establish the circumstances of yesterday’s incident.
“The six-year-old boy’s condition remains the same and officers are supporting his family.
“The male arrested is still in custody.”
On Sunday, visitors reported on social media not being allowed in or out of the gallery while emergency services dealt with the incident.
Police said a number of members of the public are assisting police with witness statements.
Administration worker Nancy Barnfield, 47, of Rochdale, was at the 10th-floor viewing gallery with a friend and their children when her friend heard a “loud bang”.
Ms Barnfield turned around and saw a woman screaming “where’s my son, where’s my son?”
Members of the public quickly gathered around a man who was nearby, she said.
Ms Barnfield said: “We did not notice the mum before, we noticed her after because she was hysterical by then.”
She said the person who was restrained by members of the public before the police arrived “just stood there and was quite calm”.
Most visitors only discovered that someone had been injured as they tried to leave.
The Tate Modern was the UK’s most popular tourist attraction in 2018 after being visited 5.9 million times, according to the Association of Leading Visitor Attractions.
With the schools having broken up and holiday season begun, there are likely to have been thousands of visitors to the gallery on Sunday.