Police investigating after a six-year-old child was thrown from a viewing platform at Tate Modern have asked witnesses who saw a male whose “behaviour seemed out of place, suspicious or worrying” to come forward.

A 17-year-old suspect has been arrested on suspicion of murder after the incident, which left the victim in a critical condition but “no longer in a life-threatening situation.”

In a statement, Detective Chief Inspector John Massey said: “We are providing around the clock support to the little boy’s family as they wait for more news on his condition and our priority remains to determine what led to yesterday’s events.

“We are grateful for the support of the public, some of whom detained the male arrested in the immediate aftermath of the incident. He was arrested by officers very quickly afterward.

“This was a truly shocking incident, and people will understandably be searching for answers.”

Nancy Barnfield was at the gallery with her two young sons during the incident.

The 47-year-old told the Mail Online she had felt threatened by his presence and had warned her children to avoid him.

She said: “He followed us around everywhere. I told my kids to stay away from that man, it went on for ages. He was acting so weird. He had his hands behind his back the whole time, his back to the wall, just watching people. We walked away from him then immediately when he was out of sight a woman was screaming ‘My son, my son’ and people were grabbing him.”

Det Ch Insp Massey added: “At the moment, this is being treated as an isolated event with no distinct or apparent motive. There is no link between the victim and male arrested.

“It would have been incredibly distressing to watch, and it may be that you left Tate Modern very quickly after. If you have not yet spoken to us about what you saw, please contact us without delay.

“My team is also very keen to talk to you if you witnessed a male whose behaviour seemed out of place, suspicious or worrying, in the hour or two before the incident in or near the gallery. It may be that this is something which has only just come back to you. If so, please still make that call to the investigation team.”

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.

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.

Malehevska said she was amazed at how quickly the emergency services arrived. She pointed out that it would not be possible for a child to climb the fence on the platform or jump through it.